Semiconductor substrate, method of manufacturing the semiconductor substrate, semiconductor device and pattern forming method

ABSTRACT

A semiconductor substrate comprises a semiconductor layer comprising a group III nitride as a main component. A scattering portion for scattering an incident beam of light incident on one plane of the semiconductor layer is provided on another plane or inside of the semiconductor layer.

BACKGROUND OF THE INVENTION

The present invention relates to a nitride semiconductor substrate to beused as a substrate for a blue light-emitting diode or a bluesemiconductor laser device or the like, a method of manufacturing thesemiconductor substrate, a semiconductor device employing the nitridesemiconductor substrate, and a pattern forming method for themanufacture of the semiconductor device.

Conventionally, semiconductor devices such as a blue light-emittingdiode (blue LED) or a blue semiconductor laser device employing a groupIII nitride such as GaN (gallium nitride), InN (indium nitride), AlN(aluminum nitride), or their mixed crystals, have been in most casesformed on a sapphire substrate.

In the manufacturing process of the semiconductor devices employing thenitride semiconductor, particularly in the manufacturing process ofsemiconductor laser devices, registration errors on the order of 1 μm donot pose any practical problems. Accordingly, a sufficient registrationaccuracy can be ensured by using inexpensive exposure apparatus (costingabout ten thousand yen per unit) using g-line (wavelength 436 nm) ori-line (wavelength 365 nm) of a mercury lamp, instead of the expensiveKrF steppers (costing several billion yen per unit), which are used inthe photolithography process for Si (silicon).

However, there was a problem that with an increasing use of a nitridesemiconductor substrate as a substrate for the semiconductor device, theaccuracy of the resist pattern (hereinafter referred to as a patternaccuracy) drops in the photolithography step during the formation of thesemiconductor device, particularly when the pattern is formed by theexposure apparatus using the g- or i-line of the mercury lamp, therebysignificantly lowering the yield of the semiconductor device.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to improve the patternaccuracy in the photolithography step during the manufacture of thesemiconductor device using the nitride semiconductor substrate.

To achieve this object, the present inventors analyzed the cause ofdeterioration in the pattern accuracy during the pattern formation bythe g- or i-line when the conventional nitride semiconductor substrateis used. The analysis revealed the following facts.

FIG. 23 illustrates the exposure of the resist film formed on aconventional nitride semiconductor substrate, specifically a substratemade from GaN (hereinafter referred to as a GaN substrate).

As shown in FIG. 23, a resist film 2 on a GaN substrate 1 is irradiatedwith an exposure light beam 4 such as the i-line through a photomask 3with an opening 3 a. The wavelength of light that can be absorbed by thenitride semiconductor is short, such as no more than 360 nm in the caseof GaN. Accordingly, when the g- or i-line is used as the exposure lightbeam 4, the exposure light beam 4 that is incident on the surface of theGaN substrate 1 through the resist film 2, i.e., an incident light beam4, propagates through the GaN substrate 1 without being absorbed. As aresult, the incident light beam 4 splits into an emitted light 5 emittedfrom the back surface of the GaN substrate 1 and a reflected light 6 dueto the reflection by the back surface of the GaN substrate 1. When theback surface of the GaN substrate 1 is specular, its reflectance withrespect to the incident light beam 4, i.e., the reflectance of theinterface between the GaN substrate 1 and air with respect to theincident light beam 4, is as much as about 20%.

A region 2 a of the resist film 2 is the region to be exposed by theincident light beam 4. However, as the resist film 2 a is exposed frombelow by the reflected light beam 6, a region 2 b of the resist film 2which is not to be exposed is also exposed. This results in defects suchas a peeling of the resist film 2 or a reduction in the resist patternsize, thereby preventing a correct pattern formation in the case ofusing the conventional nitride semiconductor substrate.

It was also found that the problem of deterioration in the patternaccuracy is pronounced when the intensity of the reflected light beam 6is increased by a reduction in the thickness of the GaN substrate 1which makes it easier for the incident light beam 4 to pass through theGaN substrate 1, or when the opening width of the opening 3 a of thephotomask 3 is not more than several times the wavelength of theincident light beam 4 or exposure light beam, in which case the incidentlight beam 4, after passing through the opening 3 a, is diffractedtowards the outside of the opening 3 a, with the reflected light beam 6being extended further outside (see FIG. 23).

It should be noted that in the present specification, the term“reflection” means specular reflection (angle of incidence=angle ofreflection), and the term “reflectance” means specular reflectance.Reflections other than the specular reflection are referred to as“diffuse reflections”. The term “substrate surface” refers to thesurface on which a nitride semiconductor layer is grown during themanufacture of the semiconductor device using the nitride semiconductorsubstrate.

Based on the above-mentioned findings, the present invention provides afirst semiconductor substrate comprising a semiconductor layer having agroup III nitride as a main component, wherein a scattering portion forscattering an incident beam of light entering the semiconductor layerthrough one plane thereof is provided on another plane or inside of thesemiconductor layer.

In the first semiconductor substrate of the invention, the scatteringportion for scattering the incident beam of light entering thesemiconductor layer from one plane thereof is provided at another planeor inside the semiconductor layer, the semiconductor layer forming thesubstrate and having a group III nitride as the main component.Accordingly, the intensity of the reflected beam of light created by thereflection of the incident light beam by the another plane can bereduced. This prevents the problem of exposing a region of the resistfilm that is not to be exposed by the exposure light beam enteringthrough the one plane (hereinafter sometimes referred to as a substratesurface) and reflected by the another surface (hereinafter sometimesreferred to as a substrate back surface), in a photolithography step forthe manufacture of a semiconductor device using the first semiconductorsubstrate, i.e., a nitride semiconductor substrate. Thus the patternaccuracy in the photolithography step can be increased and therefore themanufacturing yield of the nitride semiconductor device can be improved.For example, if the first semiconductor substrate is a GaN substrate,particularly the reflection of the g- or i-line of the mercury lamp canbe surely prevented, so that the pattern accuracy in thephotolithography step using the g- or i-line as the exposure light beamcan be significantly improved, with a resultant significant improvementin the manufacturing yield of the nitride semiconductor device.

In the first semiconductor substrate of the invention, the scatteringportion preferably may comprise height irregularity formed on theanother plane of the semiconductor layer, the height irregularity havinga height difference of 1/10 or more of the wavelength of the incidentbeam of light.

This makes the incident beam of light efficiently diffused, i.e.,scattered, on the another plane, thereby reducing the reflectance of theanother plane against the incident beam of light and thus surelyreducing the intensity of the reflected light.

In another embodiment of the invention, the reflectance of the anotherplane of the semiconductor layer against the incident beam of light ispreferably 13% or less, and the wavelength of the incident beam of lightis preferably 365 nm (i-line) or 436 nm (g-line).

In yet another embodiment of the invention, the scattering portion ispreferably provided inside the semiconductor layer and includesparticles or layer of a material having a different index of refractionthan that of the group III nitride with respect to the incident beam oflight.

In this embodiment, since the incident beam of light can be efficientlyscattered inside the semiconductor layer, the intensity of the reflectedbeam of light can surely be reduced.

The diameter of each particle of the material, the width of the layer ofthe material in a direction parallel to the one plane, or the thicknessof the layer of the material, are in each case about 1/10 or more of thewave of the incident beam of light. It is also preferable that theparticles or layer of the material are provided in a direction parallelto the one plane of the semiconductor layer, that the scattering portionis provided in a direction parallel to the one plane of thesemiconductor layer, and that the scattering portion includes anothersemiconductor layer having the group III nitride as a main component andthe particles or layer stacked alternately. The scattering portionpreferably has a thickness of about 1/10 or more of the wave of theincident beam of light. The above-mentioned material is preferably Si,SiO₂, SiN or Al₂O₃. The scattering portion preferably has atransmittance of 80% or less with respect to the incident beam of light,and the wave of the incident beam of light is preferably 365 nm or 436nm.

A second semiconductor substrate according to the present inventioncomprises a semiconductor layer having a group III nitride as a maincomponent, wherein a transmitting portion for transmitting an incidentbeam of light entering the semiconductor layer from one plane thereof isprovided on another plane of the semiconductor layer.

In accordance with the second semiconductor substrate, the transmittingportion for transmitting the incident beam of light entering thesemiconductor layer forming the substrate from the one plane thereof isprovided on the another plane of the semiconductor layer, thesemiconductor layer having a group III nitride as a main component.Accordingly, the reflectance of the another plane with respect to theincident beam of light can be reduced, whereby the intensity of thereflected light caused by the reflection of the incident beam of lightby the another plane. Thus, the problem of exposing a region of theresist film that is not to be exposed by the exposing beam of lightentering from the one plane (substrate surface) and reflected by theanother plane (substrate back surface) can be prevented in thephotolithography step in the manufacture of a semiconductor device usingthe second semiconductor substrate or nitride semiconductor substrate.The pattern accuracy in the photolithography step can therefore beimproved and thus the manufacturing yield of the nitride semiconductordevice can be increased. For example, when the second semiconductorsubstrate is a GaN substrate, particularly the reflection of the g- ori-line of the mercury lamp by the substrate back surface can be surelyprevented. As a result, the pattern accuracy in the photolithographystep using the g- or i-line as the exposing beam of light can besignificantly improved, resulting in a significantly improvedmanufacturing yield of the nitride semiconductor device.

In the second semiconductor substrate, the transmitting portionpreferably comprises a layer formed on the another plane of thesemiconductor layer, the layer formed from a material with a differentindex of refraction than that of the group III nitride with respect tothe incident beam of light.

In this manner, the reflectance of the another plane with respect to theincident beam of light can be surely reduced.

In this case, the layer of the above-mentioned material preferably iscomposed of a plurality of layers, of which at least two have differentindexes of refraction with respect to the incident beam of light. Thematerial preferably has an index of refraction which is about 9/10 orless of that of the group III nitride with respect to the incident beamof light. The material is preferably SiO₂, SiN or Al₂O₃, a compound ofthe group III element forming the semiconductor layer and oxygen, orAl_(x)Ga_(1-x)N (0<x≦1). In the case where the material is a compound ofthe group III element forming the semiconductor layer and oxygen, themanufacturing process can be simplified as compared with the case offorming a transmitting portion newly on the substrate back surface inthe form of a film. Further, the potential mixture of an impurity intothe substrate can be prevented, thereby improving the manufacturingyield of the substrate.

In the second semiconductor substrate, the transmitting portionpreferably has a transmittance of 80% or more with respect to theincident beam of light.

By doing so, the reflectance of the another plane against the incidentlight can be surely reduced. Further, it is also preferable that thewavelength of the incident light is 365 nm or 436 nm.

In the second semiconductor substrate, it is further preferable that ascattering portion is provided either between the another plane of thesemiconductor layer and the transmitting portion or inside thesemiconductor layer for scattering the incident light.

By so doing, the incident light is first scattered by the scatteringportion and then transmitted by the transmitting portion, so that theintensity of the reflected light can be further reduced.

A third semiconductor substrate according to the invention comprises asemiconductor layer having a group III nitride as a main component,wherein an absorbing portion for absorbing the incident beam of lightentering the semiconductor layer from one plane thereof is provided atleast a part of the semiconductor layer.

In the third semiconductor substrate, the absorbing portion forabsorbing the incident beam of light entering from the one plane of thesemiconductor layer, which forms the substrate and has the group IIInitride as a main component, is provided at least a part of thesemiconductor layer. Accordingly, the intensity of the reflected beam oflight caused by the reflection of the incident beam of light on theanother plane can be reduced. Thus, in the photolithography step for themanufacture of a semiconductor device using the third semiconductorsubstrate or nitride semiconductor substrate, the problem of exposing aregion of the resist film that is not to be exposed by the exposing beamof light entering from the one plane (substrate surface) and reflectedby the another plane (substrate back surface) can be avoided. As aresult, the pattern accuracy in the photolithography step can beimproved, and therefore the manufacturing yield of the nitridesemiconductor device can be improved. For example, when the thirdsemiconductor substrate is a GaN substrate, particularly the reflectionof the g- or i-line of the mercury lamp by the substrate back surfacecan be surely prevented, so that the pattern accuracy in thephotolithography step using the g- or i-line as the exposing beam oflight can be significantly increased, thereby also improving themanufacturing yield of the nitride semiconductor device.

In the third semiconductor substrate, it is preferable that thetransmittance of the absorbing portion against the incident beam oflight is 80% or less.

By so doing, even when the substrate back surface is specular thereflectance of the substrate back surface against the incident beam oflight can be reduced to substantially 13% or lower, thereby surelyimproving the pattern accuracy in the photolithography step. Also, thewavelength of the incident beam of light is preferably 365 nm or 436 nm.

In the third semiconductor substrate, the absorbing portion ispreferable made from a material with a larger absorption coefficientthan that of the group III nitride with respect to the incident beam oflight.

By so doing, the incident beam of light can be surely absorbed by theabsorbing portion, whereby the intensity of the incident beam of lightcan be surely reduced. In this case, it is preferable that the materialis composed of a plurality of materials having different absorptioncoefficients with respect to the incident beam of light, or includes atleast one of Si and W.

In the third semiconductor substrate, the absorption portion is formedby adding an impurity to the semiconductor layer such that a levelarises that absorbs the incident beam of light.

By so doing, the incident beam of light can be surely absorbed by theabsorbing portion, so that the intensity of the reflected beam of lightcan be surely reduced, and the lowering of crystallinity of the thirdsemiconductor substrate or nitride semiconductor substrate can beprevented. Further, the impurity preferably contains at least one of C,O, Si, S, Cl, P and As. It is also preferable that the relationship z00.223/α holds where α is the absorption coefficient of the absorbingportion with respect to the incident beam of light and z0 is thethickness of the absorbing portion.

In the third semiconductor substrate, the absorbing portion preferablycomprises point defects formed in the semiconductor layer.

By so doing, the incident beam of light can be surely absorbed by theabsorbing portion, so that the intensity of the reflected beam of lightcan be surely reduced, and also the lowering of crystallinity of thethird semiconductor substrate or nitride semiconductor substrate can beprevented. In this case, the point defects are preferably formed byintroducing protons into the semiconductor layer.

In the third semiconductor substrate, the absorbing portion ispreferably distributed non-uniformly along a direction parallel to theone plane of the semiconductor layer.

By so doing, not only does the absorbing portion absorbs the incidentbeam of light but also it scatters the incident beam of light, so thatthe intensity of the reflected beam of light can be further reduced.Further, when producing a ridge-type laser device by using thesemiconductor substrate, by not providing the absorbing portion on thelower side of the ridge structure in the semiconductor substrate, thepattern accuracy in the photolithography step can be improved withoutadversely affecting the characteristics of the activation layer of thesubstrate.

A first method of manufacturing a semiconductor substrate according tothe present invention includes the steps of; partially forming a lightscattering portion on a first semiconductor layer having a group IIInitride as a main component, the light scattering portion formed from amaterial with different optical index of refraction than that of thegroup III nitride; and crystal-growing a second semiconductor layerhaving the group III nitride as a main component on the firstsemiconductor layer including the light scattering portion, whereby asemiconductor substrate comprising the first semiconductor layer, thelight scattering portion and the second semiconductor layer is formed.

In accordance with the first method of manufacturing the semiconductorsubstrate, since the light scattering portion is formed between thefirst semiconductor layer and the second semiconductor layer forming thesemiconductor substrate, the intensity of the light entering from thesubstrate surface and then reflected by the substrate back surface,i.e., a reflected beam of light can be reduced. Accordingly, in thephotolithography step for the manufacture of a nitride semiconductordevice using this semiconductor substrate, the problem of exposing aregion of the resist film that is not to be exposed can be avoided,thereby increasing the pattern accuracy and increasing the manufacturingyield of the nitride semiconductor device.

Further, in accordance with the first method of manufacturing thesemiconductor substrate, after partially forming the light scatteringportion from a material with different index of refraction than that ofthe first semiconductor layer, i.e. from a different material than thatof the first semiconductor layer, the second semiconductor layer iscrystal-grown on the first semiconductor layer including the lightscattering portion. This makes it possible to prevent the defects andthe like present in the first semiconductor layer from being conveyed tothe second semiconductor layer. Thus, an excellent crystallinity of thesecond semiconductor layer can be ensured, thereby also ensuring theexcellent crystallinity of the semiconductor substrate having the lightscattering portion.

In the first method of manufacturing the semiconductor substrate, thestep of partially forming the light scattering portion preferablycomprises the step of forming a film on the entire surface of thesemiconductor layer to serve as the optical scattering portion, the stepof partially forming a mask pattern on the film, etching the film bymeans of the mask pattern and removing the portions of the film thatwere not covered by the mask pattern, thereby forming the lightscattering portion, and the step of removing the mask pattern.

In this way, the light scattering portion can be reliably formedpartially on the first semiconductor layer.

A second method of manufacturing the semiconductor substrate accordingto the invention comprises the steps of: forming height irregularity onthe back surface of a semiconductor layer having a group III nitride asa main component, the height irregularity having a height differencelarger than a predetermined value; and forming an imbedded film on theback surface of the semiconductor layer with the height irregularityformed, the imbedded film formed from a material with a differentoptical index of refraction than that of the group III nitride, wherebya semiconductor substrate comprising the semiconductor layer and theimbedded film is formed.

In accordance with the second method of manufacturing the semiconductorsubstrate, the height irregularity forming the light scattering portionare formed on the back surface of the semiconductor layer forming thesemiconductor substrate, i.e., on the interface between thesemiconductor layer and the imbedded film. This makes it possible toreduce the intensity of the light entering from the substrate surfaceand then reflected by the substrate back surface, i.e., the reflectedbeam of light. Accordingly, in the photolithography step for themanufacture of the nitride semiconductor device using the semiconductorsubstrate, the problem of exposing the regions of the resist film thatare not to be exposed can be avoided, so that the pattern accuracy canbe increased and the manufacturing yield of the nitride semiconductordevice can be improved.

Further, in accordance with the second method of manufacturing thesemiconductor substrate, since the back surface of the semiconductorlayer which was made coarse by the height irregularity can be flattenedby the imbedded film, the substrate back surface can be flattened andthe manufacturing process of the semiconductor device can be simplified.

Further, in accordance with the second method of manufacturing thesemiconductor substrate, when another semiconductor layer having thegroup III nitride as a main component is crystal-grown as the embeddedfilm, an excellent crystallinity can be obtained in the anothersemiconductor layer formed on the convex portions of the heightirregularity. Accordingly, an excellent crystallinity can be obtained inthe semiconductor substrate having the light scattering portion.

A third method of manufacturing the semiconductor substrate according tothe invention comprises the steps of: partially forming a lightabsorbing portion on the first semiconductor layer having the group IIInitride as the main component, the light absorbing portion formed from amaterial with a larger optical absorption coefficient than that of thegroup III nitride; crystal-growing a second semiconductor layer on thefirst semiconductor layer including the light absorbing portion, thesecond semiconductor layer having the group III nitride as the maincomponent, whereby a semiconductor substrate comprising the firstsemiconductor layer, the light absorbing portion and the secondsemiconductor layer is formed.

In accordance with the third method of manufacturing the semiconductorsubstrate, the light absorbing portion is formed between the firstsemiconductor layer forming the semiconductor substrate and the secondsemiconductor layer. Accordingly, the light entering from the substratesurface and then reflected by the substrate back surface, i.e., areflected beam of light can be reduced in intensity. Thus, in thephotolithography step for the manufacture of the nitride semiconductordevice using this semiconductor substrate, the problem of exposing theregions of the resist film that are not to be exposed can be prevented,whereby the pattern accuracy can be improved and the manufacturing yieldof the nitride semiconductor device can be increased.

Further, in accordance with the third method of manufacturing thesemiconductor substrate, after partially forming the light absorbingportion from a material with different absorption coefficient than thatof the first semiconductor layer, i.e. from a different material thanthat of the first semiconductor layer, the second semiconductor layer iscrystal-grown on the first semiconductor layer including the lightabsorbing portion. Accordingly, the conveyance of the defects and thelike present in the first semiconductor layer to the secondsemiconductor layer can be prevented by the light absorbing portion.Thus, an excellent crystallinity can be obtained in the semiconductorsubstrate having the light absorbing portion.

A fourth method of manufacturing the semiconductor substrate inaccordance with the invention comprises the step of forming the lightabsorbing portion by implanting an impurity into the semiconductor layerhaving the group III nitride as the main component, thereby establishinga level that absorbs light, whereby a semiconductor substrate comprisingthe semiconductor layer and the light absorbing portion is formed.

In accordance with the fourth method of manufacturing the semiconductorsubstrate, the light absorbing portion is formed in the semiconductorlayer forming the semiconductor substrate, so that the light enteringfrom the substrate surface and then reflected by the substrate backsurface, i.e. a reflected beam of light, can be reduced in intensity.This makes it possible to prevent the problem of exposing the regions ofthe resist film that are not to be exposed in the photolithography stepfor the manufacture of the nitride semiconductor device using thissemiconductor substrate. Accordingly, the pattern accuracy can beincreased and therefore the manufacturing yield of the nitridesemiconductor device can be improved.

Further, in accordance with the fourth method of manufacturing thesemiconductor substrate, since the light absorbing portion is formed byimplanting an impurity into the semiconductor layer forming thesubstrate, the lowering of the crystallinity of the semiconductorsubstrate having the light absorbing portion can be prevented.

In accordance with the fourth method of manufacturing the semiconductorsubstrate, the step of forming the light absorbing portion preferablycomprises the step of partially forming the light absorbing portion inthe semiconductor layer by partially forming a mask pattern on thesemiconductor layer and then implanting an impurity into thesemiconductor layer with the use of the mask pattern, and the step ofremoving the mask pattern.

In this manner, the light absorbing portion can be surely formedpartially on the semiconductor layer. When a ridge-type laser device isproduced using the semiconductor substrate, for example, the patternaccuracy in the photolithography step can be improved without adverselyaffecting the characteristics of the activation layer on thesemiconductor substrate by not providing the absorbing portion on thelower side of the ridge structure of the semiconductor substrate.

A first semiconductor device according to the invention comprises asemiconductor substrate having a group III nitride as a main componentand a scattering portion for scattering light entering from one plane ofthe substrate, the scattering portion provided on another plane orinside of the substrate, and a structure formed on the one plane of thesemiconductor substrate by a photolithography and etching of thesemiconductor layer of the group III nitride.

In accordance with the first semiconductor device, which employs thefirst semiconductor substrate according to the invention, no undesiredexposure of the resist film occurs during the photolithography step. Asa result, the dimensional accuracy of the structure formed on thesubstrate can be improved and therefore the manufacturing yield of thesemiconductor device can be improved.

A second semiconductor device according to the invention comprises asemiconductor substrate having a transmitting portion for transmittinglight entering from one plane of the substrate and having a group IIInitride as a main component, the transmitting portion provided onanother plane of the substrate, and a structure formed on the one planeof the semiconductor substrate by a photolithography and etching of thesemiconductor layer of the group III nitride.

In accordance with the second semiconductor device, which employs thesecond semiconductor substrate according to the invention, no unwantedexposure of the resist film occurs during the photolithography step.Accordingly, the dimensional accuracy of the structure formed on thesubstrate can be improved and the manufacturing yield of thesemiconductor device can be improved.

A third semiconductor device according to the invention comprises asemiconductor substrate having an absorbing portion for absorbing lightentering from one plane of the substrate and having a group III nitrideas a main component, the absorbing portion provided at least a part ofthe semiconductor substrate, and a structure formed on the one plane ofthe semiconductor substrate by a photolithography and etching of thesemiconductor layer of the group III nitride.

In accordance with the third semiconductor device, which employs thethird semiconductor substrate according to the invention, no undesiredexposure of the resist film occurs during the photolithography step.Accordingly, the dimensional accuracy of the structure formed on thesubstrate can be improved and therefore the manufacturing yield of thesemiconductor device can be improved.

In any of the first to the third semiconductor device, the structure mayinclude a ridge structure or a trench structure.

In the third semiconductor device, the structure comprises a ridgestructure, and it is preferable that the absorbing portion is notprovided at the lower side of the ridge structure of the semiconductorsubstrate. In this manner, the pattern accuracy of the photolithographystep can be improved without adversely affecting the characteristics ofthe activation layer on the substrate.

A first pattern forming method according to the invention comprises thesteps of: forming a semiconductor layer of a group III nitride on oneplane of a semiconductor substrate having a scattering portion forscattering light entering from the one plane of the substrate and havingthe group III nitride as a main component, the semiconductor substrate,the scattering portion provided on another plane or inside of thesemiconductor substrate; forming a positive- or negative-type resistfilm on the semiconductor layer; irradiating the resist film with aexposing beam of light via a photomask having an opening; forming aresist pattern by developing the resist film so that, where the resistfilm is of the positive type, portions of the resist film that wereirradiated by the exposing beam of light are removed and where theresist film is of the negative type, portions of the resist film thatwere not irradiated by the beam of exposing beam of light are removed;and etching the semiconductor layer by using the resist pattern as amask.

In accordance with the first pattern forming method, which is a patternforming method for the manufacture of the semiconductor device using thefirst semiconductor substrate according to the invention, the problem ofexposing the regions of the resist film that are not to be exposed canbe prevented. Accordingly, the resist pattern accuracy can be improvedand therefore the manufacturing yield of the semiconductor device can beimproved.

A second pattern forming method according to the invention comprises thesteps of: forming a semiconductor layer of a group III nitride on oneplane of a semiconductor substrate having a transmitting portionprovided on another plane of the semiconductor substrate fortransmitting a beam of light incident from the one plane, thesemiconductor substrate comprising the group III nitride as a maincomponent; forming a positive- or negative-type resist film on thesemiconductor layer; irradiating the resist film with an exposing beamof light through a photomask having an opening portion; developing theresist film so that, where the resist film is of positive type, portionsof the resist film that were irradiated with the exposing beam of lightare removed and where the resist film is of negative type, portions ofthe resist film that were not irradiated with the exposing beam of lightare removed, whereby a resist pattern is formed; and etching thesemiconductor layer while using the resist pattern as a mask.

In accordance with the second pattern forming method, which is a patternforming method for manufacturing the semiconductor device using thesecond semiconductor substrate according to the invention, the problemof exposing the regions of the resist film that are not to be exposedcan be prevented. Accordingly, the resist pattern accuracy can beimproved and therefore the manufacturing yield of the semiconductordevice can be improved.

A third pattern forming method according to the invention comprises thesteps of: forming a semiconductor layer of a group III nitride on oneplane of a semiconductor substrate provided at least partially with anabsorbing portion for absorbing a beam of light incident from the oneplane, the semiconductor substrate comprising the group III nitride as amain component; forming a positive- or negative-type resist film on thesemiconductor layer; irradiating the resist film with an exposing beamof light through a photomask having an opening; developing the resistfilm so that, where the resist film is of positive type, portions of theresist film that were irradiated by the exposing light beam are removedand where the resist film is of negative type, portions of the resistfilm that were not irradiated by the exposing beam of light are removed,whereby a resist pattern is formed; and etching the semiconductor layerwhile using the resist pattern as a mask.

In accordance with the third pattern forming method, which is a patternforming method for the manufacture of the semiconductor device using thethird semiconductor substrate according to the invention, the problem ofexposing the regions of the resist film that are not to be exposed canbe prevented. Accordingly, the resist pattern accuracy can be improvedand therefore the manufacturing yield of the semiconductor device can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a semiconductor substrate accordingto a first embodiment of the invention;

FIGS. 2(A)-(E) are cross sectional views of the semiconductor substrateaccording to the first embodiment of the invention in the respectivesteps of its manufacture;

FIG. 3 shows an example of the MOVPE apparatus used in the method ofmanufacturing the semiconductor substrate according to the firstembodiment of the invention;

FIG. 4 shows an example of the HVPE apparatus used in the method ofmanufacturing the semiconductor substrate according to the firstembodiment of the invention;

FIG. 5 illustrates the exposure of a resist film formed on thesemiconductor substrate according to the first embodiment of theinvention;

FIG. 6 is a graph showing the relationship between the reflectance ofthe substrate back surface against the exposing beam of light and theratio of exteriorly conforming items of the resist pattern forming aline portion, in the case where a line-and-space resist pattern with aline and space portion width of 2 μm was formed on a GaN substrate;

FIG. 7 is a cross sectional view of a semiconductor substrate accordingto a second embodiment of the invention;

FIGS. 8(A)-(C) are cross sectional views of the semiconductor substrateaccording to a variation of the second embodiment of the invention inits respective manufacturing steps;

FIG. 9 is a cross sectional view of the semiconductor substrateaccording, to a third embodiment of the invention;

FIGS. 10(A)-(E) are cross sectional views of a semiconductor substrateaccording to a third embodiment of the invention in its respectivemanufacturing steps;

FIG. 11 is a cross sectional view of a semiconductor substrate accordingto a fourth embodiment of the invention;

FIG. 12 is a cross sectional view of a semiconductor substrate accordingto a fourth embodiment of the invention;

FIG. 13(A)-(E) are cross sectional views of a semiconductor substrateaccording to a fifth embodiment of the invention in its respectivemanufacturing steps;

FIG. 14(A)-(C) are cross sectional views of a semiconductor substrateaccording to a fifth embodiment of the invention in its respectivemanufacturing steps;

FIG. 15 is a cross sectional view of a semiconductor substrate accordingto a sixth embodiment of the invention;

FIG. 16(A)-(F) are cross sectional views of a semiconductor substrateaccording to a fifth embodiment of the invention in its respectivemanufacturing steps;

FIG. 17 is a cross sectional view of a semiconductor substrate accordingto a seventh embodiment of the invention;

FIG. 18(A)-(E) are cross sectional views of a semiconductor substrateaccording to a seventh embodiment of the invention in its respectivemanufacturing steps;

FIG. 19 illustrates an example of the method of implanting As into theGaN substrate in the method of manufacturing the semiconductor substrateaccording to the seventh embodiment of the invention;

FIG. 20(A)-(D) are cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor device using thesemiconductor substrate according to the seventh embodiment of theinvention;

FIG. 21(A)-(D) are cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor device using thesemiconductor substrate according to the seventh embodiment of theinvention;

FIG. 22 is a cross sectional view of the semiconductor device using thesemiconductor substrate according to the seventh embodiment of theinvention; and

FIG. 23 illustrates the exposure of the resist film formed on theconventional nitride semiconductor substrate.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

Hereunder, a semiconductor substrate according to a first embodiment ofthe invention, the method of manufacturing the semiconductor substrate,and a pattern forming method for the manufacture of the semiconductordevice using the semiconductor substrate will be described by referringto the drawings.

FIG. 1 shows a cross-section of the semiconductor substrate according tothe first embodiment of the invention.

As shown, the semiconductor substrate according to the first embodimentcomprises a group III nitride semiconductor layer, specifically a GaNlayer 100. The surface of the GaN layer 100 (which may hereunder bereferred to as a GaN substrate 100) is a (0001) Ga surface. The backsurface of the GaN substrate 100 is (0001) an N surface. The thicknessof the GaN substrate 100 is 200 μm, for example.

The first embodiment is characterized in that the back surface of theGaN substrate is coarsely formed with height irregularity 100 a. Theheight irregularity 100 a preferably has a height difference of λ/10 ormore, where λ is the wavelength of the exposing beam of light used inthe photolithography step for the manufacture of the semiconductordevice using the GaN substrate 100.

FIGS. 2(A)-(E) show cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor substrateaccording to the first embodiment of FIG. 1.

First, as shown in FIG. 2(A), there is prepared a silicon-on-sapphire(hereinafter referred to as an SOS substrate) comprising a sapphiresubstrate 101 with a thickness of 300 μm and a silicon substrate 102with a thickness of 80 μm.

Next, as shown in FIG. 2(B), an AlN layer 103 is grown on the siliconsubstrate 102 of the SOS substrate to a thickness of 20 nm under thetemperature of 1000° C. by the MOVPE (metal organic vapor phase epitaxy)method using trimethylaluminum and ammonia as raw material gases.

FIG. 3 shows an example of the MOVPE apparatus used in the manufactureof the semiconductor substrate according to the first embodiment. Asshown in FIG. 3, the MOVPE apparatus comprises a reaction pipe 150 madefrom quarts or stainless and the like, a susceptor 152 on which aprocessed substrate 151 is placed in the reaction pipe 150, and aheating means 153 for heating the processed substrate 151 via thesusceptor 152 in the reaction pipe 150. The reaction pipe 150 comprisesa gas inlet 150 a for the introduction of the raw material gas and acarrier gas, and a gas outlet 150 b for the discharge of used gases. Thesusceptor 152 is made from graphite, for example. The heating means 153may comprise a resistance wire heater (resistance heating heater) or alamp heater, for example.

During the growing of the group III nitride semiconductor layer by theMOVPE method, the group III element plane has a high growth rate, sothat the growth of the group III element is dominant. Accordingly, inthe step of forming the AlN layer 103 shown in FIG. 2(B), the growth ofthe Al plane is dominant, with the result that the surface of the AlNlayer 103 becomes an Al plane while the back surface of the AlN layer103, i.e., the plane of the AlN layer 103 facing the silicon substrate102, becomes an N plane.

Next, as shown in FIG. 2(C), a GaN layer 100 is grown on the AlN layer103 to a thickness of 250 μm under the temperature of 1000° C. by theHVPE (hydride vapor phase epitaxy) method using the gallium chlorideobtained as a result of reacting an HCl gas with Ga under thetemperature of 800° C., and ammonia as raw material gases. Since thesurface of the AlN layer 103 is an Al plane, the plane of the GaN layer100 facing the AlN layer 103, i.e., the back surface of the GaN layer100, becomes an N plane, while the surface of the GaN layer 100 becomesa Ga plane. Further, since the GaN layer 100 is formed on the SOSsubstrate including the sapphire substrate 101 and the silicon substrate102, the compressive stress given by the sapphire substrate 101 to theGaN layer 100 balances with the tensile stress given by the siliconsubstrate 102 to the GaN layer 100, so that a thick-film GaN layer 100can be grown without generating cracks.

FIG. 4 shows an example of the HVPE apparatus used in the method ofmanufacturing the semiconductor substrate according to the firstembodiment. As shown in FIG. 4, the HVPE apparatus comprises a reactionpipe 160 formed from quarts or the like, a susceptor 162 on which aprocessed substrate 161 is placed in the reaction pipe 160, a pan 164 inwhich a molten Ga 163 is placed for reaction with the HCl gas in thereaction pipe 160, and a heating means 165 for heating the inside of thereaction pipe 160 externally. The reaction pipe 160 comprises a firstgas inlet 160 a for the introduction of ammonia gas and carrier gas, asecond gas inlet 160 b for the introduction of the HCl gas and thecarrier gas, and a gas outlet 160 c for the discharge of used gases. Thesusceptor 162 is made from graphite or quarts, for example, and the pan164 is made from quarts, for example. The heating means 165 may comprisea tubular resistance-wire heater, for example.

Next, as shown in FIG. 2(D), the sapphire substrate 101 is separatedfrom the GaN substrate 10, i.e., the GaN layer 100 which is to form thenitride semiconductor substrate, by removing only the silicon substrate102 by a processing with the use of a liquid mixture of hydrofluoricacid and nitric acid. Either side of the GaN substrate 100 is specularwithout any height irregularity, and the AlN layer 103 is formed on theback surface of the GaN substrate 100.

The AlN layer 103 is removed by grinding the back surface of the GaNsubstrate 100, as shown in FIG. 2(E), and the back surface is then madecoarse by forming the height irregularity 10 a. The coarsening of theback surface of the GaN substrate 100 can be done by various methods,including, for example, polishing the back surface of the GaN substrate100 by using an abrasive with a grain size of 10-50 μm, for example.Specifically, in the first embodiment, the back surface of the GaNsubstrate 100 is polished down to a thickness of the GaN substrate 100on the order of 200 μm, in order to obtain the semiconductor substrateaccording to the first embodiment of FIG. 1.

FIG. 5 illustrates the exposure of the resist film formed on thesemiconductor substrate according to the first embodiment of FIG. 1,i.e. the GaN substrate 100.

As shown in FIG. 5, the resist film 171 on the GaN substrate 100 isirradiated with an exposing beam of light 173 of g-line, for example,via a photomask 172 having an opening 172 a. The exposing beam of light173 or incident beam of light 173 incident on the surface of the GaNsubstrate 100 (which may hereinafter be referred to as a substratesurface) through the resist film 171 splits into an emergent beam oflight 174 emerging from the back surface of the GaN substrate 100 (whichmay hereinafter be referred to as a substrate back surface) and areflected beam of light 175 caused by the diffusion by the heightirregularity 100 a on the substrate back surface. A region 171 a of theresist film 171 is the region to be exposed by the incident beam oflight 173. The exposing apparatus may comprise a contact aligner usingthe g-line of a mercury lamp as the light source.

The present inventors formed resist films with various back-surfaceshapes on the GaN substrate, including the semiconductor substrateaccording to the first embodiment, formed resist patterns by developingthe resist films and examined the exterior features of the resistpatterns. Hereunder, the results of the examination will be described byreferring to FIG. 6.

FIG. 6 shows the relationship between the reflectance of the exposingbeam of light (g-line) on the substrate back surface and the ratio ofexteriorly conforming items of the resist pattern forming the lineportion, in the case where a line-and-space resist pattern with a lineand space portion thickness of 2 μm is formed on the GaN substrate.

The exteriorly conforming item in this case refers to those resistpatterns having a width of 2±0.2 μm. The reflectance of the exposingbeam of light on the substrate back surface (hereinafter referred to asa back surface reflectance) is varied by varying the grain diameter ofthe abrasive or the polishing time in the polishing step for thecoarsening of the back surface. The back surface reflectance isindicated as a ratio (measurement value) of the intensity of thereflected beam of light emerging from the substrate surface at anemerging angle of 90° (or substantially 90°) after reflection by thesubstrate back surface, to the intensity of the incident beam of lightincident on the substrate surface at an incident angle of 90° (orsubstantially 90°). For example, when the back surface of the GaNsubstrate is so flat that the steps on the atomic layer order can beobserved by an atomic force microscope, the back surface reflectance ison the order of 21%. Such an extremely flat surface can be obtained bypolishing the back surface of the GaN substrate with an abrasive with anextremely fine grain diameter of less than 1 μm and then heating theback surface to a temperature of about 1000° C. in an atmosphere ofammonia.

As shown in FIG. 6, when the back surface reflectance is 16% or more,the resist film is exposed from below by an intense reflected beam oflight, resulting in a peeling of the resist film or a reduction in thesize of the resist pattern and thus a lowering of the ratio ofexteriorly conforming items. On the other hand, as the back surfacereflectance is lowered, the ratio of exteriorly conforming itemsincreases, and a ratio of exteriorly conforming items of substantially100% is achieved as the back surface reflectance is lowered to 13% orless. Accordingly, by lowering the back surface reflectance to 13% orless, desired resist patterns can be formed with an yield ofsubstantially 100%.

It will be explained hereunder how the results of FIG. 6 are obtained.When the back surface of the GaN substrate is specular, the back surfacereflectance reaches about 20% or more as mentioned above (see theSUMMARY OF THE INVENTION section or FIG. 21). As a result, the resistfilm is exposed from below by the reflected beam of light from thesubstrate back surface during the exposure, giving rise to such defectsas a peeling of the resist film or a reduction in the resist patternsize. In particular, when the width of the opening of the photomask isnot more than several times the wavelength of the exposing beam oflight, the reflected beam of light extends somewhat beyond the openingdue to the diffraction of the exposing beam of light by the photomask,making it more likely that the region of the resist film that is not tobe exposed is exposed. In contrast, when the back surface of the GaNlayer forming the substrate is made coarse as in the present embodiment,diffusion is caused by the substrate back surface during the exposure,namely it is difficult for specular reflection to occur on the substrateback surface. This makes the back surface reflectance to decrease, sothat there is little unwanted exposing light coming from the back of theresist film, making possible the formation of a resist pattern with noexterior defects.

Thus, in accordance with the first embodiment, because the heightirregularity 100 a is formed on the back surface of the GaN substrate100 to make the back surface coarse, it is difficult for specularreflection of the incident beam of light 173 to occur on the backsurface of the GaN substrate 100 and the back surface reflectance can besurely reduced. Accordingly, the intensity of the reflected beam oflight 175 decreases, so that the problem of exposing the regions (i.e.,regions other than the region 171 a) of the resist film 171 that is notto be exposed by the reflected beam of light 175 can be avoided. Hencethe pattern accuracy of the resist pattern formed by the resist film 171can be increased and also the pattern accuracy in the photolithographystep for the manufacture of the semiconductor device using the GaNsubstrate 100 can be improved. Thus, the manufacturing yield of thenitride semiconductor device can be improved.

It should be noted that in the first embodiment, while the heightirregularity 100 a was formed on the back surface of the GaN substrate100 to make the back surface coarse and thereby reduce the back surfacereflectance, the back surface reflectance can be reduced by other means.For example, a dielectric substance may be sprayed non-uniformly on thesubstrate back surface, or spherical or otherwise irregularly shapedsubstance may be provided on the substrate back surface. Alternatively,a low-reflectance film may be formed on the substrate back surface. Itshould be noted, however, that the lowering of the back surfacereflectance by those means tends to be a cause for the introduction ofimpurities into various elements and the like during the subsequentsemiconductor process. Accordingly, it is preferable to lower the backsurface reflectance by the coarsening of the back surface of the GaNsubstrate 100.

Furthermore, while in the first embodiment the coarsening of the backsurface of the GaN substrate 100 was produced by polishing, this can bereplaced by other means such as sandblasting or etching.

Moreover, the first embodiment is based on the realization that theproblem of unwanted exposure of the resist film is caused by theexposing beam of light entering the substrate surface at an incidentangle of 90° (or substantially 90°), reflected by the substrate backsurface and eventually emerging from the substrate surface at anemerging angle of 90° (or substantially 90°). Accordingly, the manner ofcoarseness of the substrate back surface may be arbitrary as long as itcan reduce the reflectance of the substrate back surface with respect tothe exposing beam of light entering the substrate surface at an incidentangle of 90°. The substrate back surface, however, should preferablyhave such coarseness as to produce diffusion. This is because of thepossibility that, if the coarseness of the substrate back surface issuch that the exposing beam of light entering the substrate surface atan incident angle of 90° is reflected at a particular angle with respectto the substrate surface or substrate main plane, regions other than apredetermined exposure region of the resist film may be exposed.Specifically, the exposing beam of light entering the substrate surfaceat an incident angle of 90° can be scattered in all directions ordiffused by providing height irregularity on the substrate back surfacewith a height difference of about 1/10 or more of the wavelength of theexposing beam of light. It is also preferable that the heightirregularity provided on the substrate back surface is such that theback surface reflectance is about 13% or less.

Further, the kind of the exposing beam of light used in thephotolithography step is not particularly limited. However, asignificantly improved pattern accuracy can be obtained as compared withthe prior art by using light with certain wavelengths that can propagatethrough the GaN substrate 100 without being absorbed, such as the g- ori-line, as the exposing beam of light.

In the first embodiment, the resist film used in the photolithographystep may be either positive or negative type.

Also in the first embodiment, while GaN was used as the material for thenitride semiconductor substrate, any group III nitride semiconductormade from GaN, InN and AlN either individually or in combination as amixed crystal may be used. The substrate may include other substances aslong as such a group III nitride semiconductor as mentioned aboveconstitutes the main component of the substrate.

Embodiment 2

A semiconductor substrate according to a second embodiment of theinvention, a method of manufacturing the semiconductor substrate, and apattern forming method for the manufacture of a semiconductor deviceusing the semiconductor substrate will hereinafter described byreferring to the drawings.

FIG. 7 is a cross sectional view of the semiconductor substrateaccording to the second embodiment.

As shown, the semiconductor substrate according to the second embodimentcomprises a group III nitride semiconductor layer, specifically a GaNlayer 100 (which may hereinafter be referred to as a GaN substrate 100).The surface of the GaN substrate 100 is a (0001) Ga plane. The backsurface of the GaN substrate 100 is (0001) an N plane. The thickness ofthe GaN substrate 100 is 300 μm, for example.

The semiconductor substrate according to the second embodiment ischaracterized in that an aluminum oxide (Al₂O₃) layer 104 with athickness of 200 nm is provided on the back surface of the GaN substrate100 to function as an anti-reflection film against the exposing beam oflight of the g- or i-line. The aluminum oxide layer 104, as opposed tosapphire and the like, has a polycrystalline or an amorphous structure,while its index of refraction is 1.68, which is about the same as thatof sapphire. To serve as an anti-reflection film means that it preventsthe reflection of the incident beam of light entering from the surfaceof the GaN substrate 100 by the back surface thereof, while allowing thetransmission of the light through the anti-reflection film to emergefrom the back surface of the film Thus, the anti-reflection filmpreferably has a transmittance of 80% or more.

The method of manufacturing the semiconductor substrate according to thesecond embodiment of FIG. 7 is as follows. It should be noted that thismethod is substantially the same as that for the first embodiment up tothe step shown in FIG. 2(D) of FIGS. 2(A)-(E).

Specifically, as shown in FIG. 2(A), first there is prepared the SOSsubstrate including the sapphire substrate 101 and the silicon substrate102, and then, as shown in FIG. 2(B), the AlN layer 103 is grown on thesilicon substrate 102 of the SOS substrate to the thickness of 200 nm bythe MOVPE method using the trimethylaluminum and ammonia as the rawmaterial gases. Thereafter the GaN layer 100 is grown on the AlN layer103 to a thickness of 300 μm by the HVPE method using the galliumchloride and ammonia as the raw material gases, as shown in FIG. 2(C).This is followed by the removal of only the silicon substrate 102, asshown in FIG. 2(D), whereby the sapphire substrate 101 is separated fromthe GaN layer 100 forming the nitride semiconductor substrate, i.e. theGaN substrate 100. Either side of the GaN substrate 100 is specular andwithout any height irregularity, while on the back surface of the GaNsubstrate 100 is formed the AlN layer 103.

Next, the GaN substrate 100 including the AlN layer 103 is heated at atemperature of 600° C. for ten minutes in an atmosphere of water vaporand nitrogen with the partial pressures of 10% and 90%, respectively, atatmospheric pressure. This allows the AlN layer 103 to be selectivelyoxidized to form an aluminum oxide layer 104 with a thickness of 200 nm.Thus the semiconductor substrate according to the second embodimentshown in FIG. 7 is completed.

The present inventors formed a line-and-space resist pattern similar tothe one of first embodiment on the semiconductor substrate according tothe second embodiment via a processed film by exposure using the g-line,and then etched the processed film using the resist pattern as the mask.This resulted in the size of the patterned processed film fallingsubstantially completely within a predetermined range. The examinationof the reflectance of the back surface of the semiconductor substrateaccording to the second embodiment revealed a very low value of about0.5% or less, which presumably accounted for the satisfactory formationof the pattern.

Specifically, in accordance with the second embodiment, the aluminumoxide layer 104 formed on the back surface of the GaN substrate 100functions as the anti-reflection film and therefore the back surfacereflectance decreases, whereby the intensity of the light entering fromthe substrate surface and then reflected by the substrate back surfacecan be reduced. Accordingly, the problem of exposing the regions of theresist film other than the predetermined exposed regions by the exposingbeam of light incident on the substrate surface and reflected by thesubstrate back surface during the photolithography step for themanufacture of the semiconductor device using the GaN substrate 100.Thus the pattern accuracy in the photolithography step can be improvedand therefore the manufacturing yield of the nitride semiconductordevice can be increased.

Further, in accordance with the second embodiment, the aluminum oxidelayer 104 is formed by oxidizing the AlN layer 103, which had beenformed on the back surface of the GaN substrate 100 when the GaNsubstrate 100 was formed. As a result, the process can be simplified ascompared with the case where the anti-reflection film is newly formed onthe back surface of the GaN substrate 100, and also the manufacturingyield of the GaN substrate 100 can be improved because the possibleintroduction of impurities into the GaN substrate too is prevented.

While in the second embodiment the aluminum oxide was used as thematerial for the anti-reflection film, other materials such as SiO₂ orSiN may be used instead. Some conditions for realizing a low-reflectanceanti-reflection film with the use of those materials follow. First, thethickness of the anti-reflection film should preferably be an oddmultiple of the ¼ wavelength of the wavelength of the exposing beam oflight in the anti-reflection film. Second, the index of refraction ofthe anti-reflection film should preferably be different from that of thenitride semiconductor substrate (the GaN substrate in the presentembodiment), and this difference in the index of refraction shouldpreferably be as great as possible. Particularly, the index ofrefraction of the anti-reflection film should preferably be not morethan about 9/10 that of the nitride semiconductor substrate. Forexample, in the present embodiment, the index of refraction of thealuminum oxide layer 104 constituting the anti-reflection film is 1.68,and the thickness 200 nm of the aluminum oxide layer 104 is equivalentto three fourth the wavelength of the g-line in the anti-reflection film(436 nm/1.68). Since the difference in the indexes of refraction betweenthe aluminum oxide layer 104 (1.68) and the GaN substrate 100 (about2.5) is relatively large, the tolerable range of the thickness forobtaining a back surface reflectance of 13% or less, i.e., the tolerablerange of the thickness for ensuring an improvement in the patternaccuracy during the photolithography step, is relatively wide.Specifically, when the thickness of the aluminum oxide layer 104 is setby targeting the ¼ wavelength of the wavelength of the exposing beam oflight in the aluminum oxide layer 104, the tolerable thickness range forproducing the back surface reflectance of 13% or less is about the ¼wavelength±⅛ wavelength. This is, in the case where the g-line is usedfor exposure, about 65 nm±40 nm.

Tables 1 and 2 show the characteristics of various anti-reflection filmsexamined by the inventors, where the i- and g-lines were used as theexposing beam of light. It should be noted that those characteristicswere in each case obtained when the thickness of the anti-reflectionfilm was set by targeting the ¼ wavelength of the wavelength of theexposing beam of light in the anti-reflection film and when the GaNsubstrate was used as the nitride semiconductor substrate. TABLE 1 Rangeof Film Reflectance film thickness against i- thickness correspondingline when that can Index of to ¼ film produce refraction wavelength ofthickness index of Type of against i- i-line in corresponds refractionanti- line anti- to ¼ of 13% or reflection (wavelength reflectionwavelength less against film 365 nm) film of i-line i-line SiO₂ 1.47 62nm 0.5% 25-99 nm Al₂O₃ 1.68 54 nm 0.4% 22-86 nm SiN 2.05 45 nm 6.5%22-67 nm ZrO₂ 2.12 43 nm 8.2% 23-63 nm Al_(0.5)Ga_(0.5)N 2.32 39 nm  13%38-41 nm Al_(0.1)Ga_(0.9)N 2.45 37 nm  17% None TiO₂ 2.70 33 nm  24%None

TABLE 2 Range of Film Reflectance film thickness against g- thicknesscorresponding line when that can Index of to ¼ film produce refractionwavelength of thickness index of Type of against g- g-line incorresponds refraction anti- line anti- to ¼ of 13% or reflection(wavelength reflection wavelength less against film 436 nm) film ofg-line g-line SiO₂ 1.47 74 nm 0.5% 30-118 nm  Al₂O₃ 1.68 65 nm 0.4%26-104 nm  SiN 2.05 53 nm 6.5% 26-80 nm ZrO₂ 2.09 52 nm 9.3% 25-78 nmAl_(0.5)Ga_(0.5)N 2.32 47 nm  13% 45-49 nm Al_(0.1)Ga_(0.9)N 2.44 45 nm 17% None TiO₂ 2.45 44 nm  17% None

As will be seen from Tables 1 and 2, no satisfactory effect as theanti-reflection film can be obtained unless the index of refraction ofthe anti-reflection film is made not more than 2.32, which is smallerthan the index of refraction of GaN (2.5). Further, as the index ofrefraction of the anti-reflection film becomes smaller than that of GaN,the reflectance (back surface reflectance) reducing effect, i.e., theanti-reflection effect, becomes larger while simultaneously expandingthe tolerable range of the thickness for making the back surfacereflectance 13% or less. It should be noted that in Tables 1 and 2, thefact that the index of refraction of the anti-reflection film in thecases of TiO₂ and ZrO₂ varies depending on whether the g-line or i-lineis used is due to a larger wavelength dispersion near the wavelengths ofthe g- and i-lines in the cases of TiO₂ and ZrO₂.

In the second embodiment, the anti-reflection film may be formed by alaminated member composed of a plurality of layers with differentindexes of refraction than that of the group III nitride forming thesubstrate. In this case, it is preferable that at least two of thelayers forming the laminated member have different indexes ofrefraction.

Further, in the second embodiment, the type of the exposing beam oflight used in the photolithography step is not particularly limited.However, by using light with certain wavelengths that can propagatethrough the GaN substrate 100 without being absorbed, such as the g- ori-line, as the exposing beam of light, the pattern accuracy can begreatly increased as compared with the prior art.

In the second embodiment, the resist film used in the photolithographystep may be either positive or negative type.

While in the second embodiment, GaN was used as the material for thenitride semiconductor substrate, any group III nitride semiconductormade from GaN, InN and AlN either individually or in combination as amixed crystal may be used. In this case, the substrate may include othermaterials as long as the group III nitride as mentioned aboveconstitutes the main component of the substrate.

(Variation of the Second Embodiment)

Hereunder a semiconductor substrate according to a variation of thesecond embodiment and a method of manufacturing the same will bedescribed by referring to the drawings.

The variation of the second embodiment differs from the secondembodiment in that a gallium oxide (Ga₂O₃) layer is used as theanti-reflection film against the exposing beam of light of g- or i-line,instead of the aluminum oxide layer.

FIGS. 8(A)-(C) are cross sectional views illustrating the respectivesteps of the method of manufacture of the semiconductor substrateaccording to the variation of the second embodiment. This manufacturingmethod is substantially the same as that for the first embodiment asshown in FIGS. 2(A)-(E) except for the step shown in FIG. 2(E).

Specifically, as shown in FIG. 2(A), there is first prepared the SOSsubstrate including the sapphire substrate 101 and the silicon substrate102. This is followed by the growing of the AlN layer 103 on the siliconsubstrate 102 of the SOS substrate to the 200 nm thickness by the MOVPEmethod using trimethylaluminum and ammonia as the raw material gases, asshown in FIG. 2(B). Thereafter the GaN layer 100 is grown on the AlNlayer 103 to the 300 μm thickness by the HVPE method using galliumchloride and ammonia as the raw material gases, as shown in FIG. 2(C).The silicon substrate 102 is then removed as shown in FIG. 2(D), therebyseparating the sapphire substrate 101 from the GaN layer 100 forming thenitride semiconductor substrate, i.e. the GaN substrate 100.

Next, as shown in FIG. 8(A), the AlN layer 103 is removed by polishingwhile the back surface of the GaN substrate 100 is made specular bypolishing. Thereafter the GaN substrate 100 is heated at a temperatureof 700° C. for ten minutes in an atmosphere of oxygen and nitrogen withthe partial pressure of 10% and 90%, respectively, at atmosphericpressure. This allows either side of the GaN substrate 100 to beoxidized, thereby forming a gallium oxide layer 105 with a thickness of70 nm on the either side, as shown in FIG. 8(B).

Since the formation of the gallium oxide layer 105 on either side of theGaN substrate 100 adversely affects the subsequent device processings,the gallium oxide layer 105 on the surface side of the GaN substrate 100is removed by the following manner. Specifically, the GaN substrate 100with the gallium oxide layer 105 on either side is introduced into acrystal growing apparatus such as an MOVPE apparatus shown in FIG. 3.The back surface of the GaN substrate 100 is placed in close contactwith the susceptor such that there is no gap between the back surface ofthe GaN substrate 100 and the susceptor. The GaN substrate 100 is thenheated to a temperature of 1000° C. in an atmosphere of hydrogen. Thisallows the gallium oxide layer 105 on the surface side of the GaNsubstrate 100 to be reduced while Ga is sublimed, thereby exposing aclean GaN plane on the surface of the GaN substrate 100. On the otherhand, the gallium oxide layer 105 on the back surface side of the GaNsubstrate 100, which is in close contact with the susceptor andtherefore tends not to contact hydrogen, is hardly reduced or sublimed.As a result, as shown in FIG. 8(C), the GaN substrate 100 having thegallium oxide layer 105 on the back surface, i.e., the semiconductorsubstrate according to the variation of the second embodiment, can beobtained.

In accordance with the variation of the second embodiment, since thegallium oxide layer 105 is formed on the back surface of the GaNsubstrate 100 to function as the anti-reflection film, the back surfacereflectance can be reduced and therefore the intensity of the lightincident on the substrate surface and then reflected by the substrateback surface can be reduced. Accordingly, the problem of exposing theregions of the resist film other than the predetermined exposure regionsby the exposing beam of light incident on the substrate surface andreflected by the substrate back surface can be avoided during thephotolithography step for the manufacture of the semiconductor deviceusing the GaN substrate 100. Thus the pattern accuracy in thephotolithography step can be improved and therefore the manufacturingyield of the nitride semiconductor device can be increased.

Further, in accordance with the variation of the second embodiment, theformation of the gallium oxide layer 105 by the oxidation of the GaNsubstrate 100 makes it possible to simplify the process as compared withthe case where the anti-reflection film is newly formed on the backsurface of the GaN substrate 100. At the same time, the manufacturingyield of the GaN substrate 100 can be increased because the possibleintroduction of impurities into the GaN substrate 100 is prevented.

Furthermore, in accordance with the variation of the second embodiment,the gallium oxide layer 105 can be formed on the surface side of the GaNsubstrate 100 prior to the formation of a device using the GaN substrate100, so that the surface of the GaN substrate 100 can be protected bythe gallium oxide layer 105. When forming the device using the GaNsubstrate 100, the gallium oxide layer 15 on the surface side of thesubstrate can be reduced and removed in the crystal growing apparatus tothereby expose a clean GaN plane. This makes it possible to sequentiallyform device structures such as a cladding layer and an active layer onthe clean GaN plane.

Embodiment 3

Hereunder a semiconductor substrate according to a third embodiment ofthe invention, a method of manufacturing the semiconductor substrate,and a pattern forming method for the manufacture of a semiconductordevice using the semiconductor substrate will be described by referringto the drawings.

FIG. 9 shows a cross sectional view of the semiconductor substrateaccording to the third embodiment.

As shown in FIG. 9, the semiconductor substrate according to the thirdembodiment of the invention comprises a group III nitride semiconductorlayer, specifically a GaN layer 100. The surface of the GaN layer 100(which may hereafter referred to as a GaN substrate 100) is a (0001) Gaplane, while the back surface of the GaN substrate 100 is an (0001) Nplane. The thickness of the GaN substrate 100 is 300 μm, for example.

The third embodiment is characterized in that a plurality of SiO₂ grains106 a of a material such as SiO₂, which has a different index ofrefraction than that of GaN with respect to an exposing beam of lightsuch as g- or i-line, are discontinuously embedded in a region of theGaN substrate 100 50-80 μm below the surface. The plurality of SiO₂grains 106 a form a light scattering portion (which scatters the lightincident on the substrate surface). The shape of each SiO₂ grain 106 ais not particularly limited, but in the third embodiment, the diameterof each SiO₂ grain 106 a (on a plane parallel to the surface of the GaNsubstrate 100) is on the order of several tens of a micrometer, and itsheight (along a direction perpendicular to the surface of the GaNsubstrate 100) is on the order of 100 nm.

FIGS. 10(A)-(E) are cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor substrateaccording to the third embodiment, i.e., the semiconductor substrateshown in FIG. 9. This method of manufacturing the semiconductorsubstrate according to the third embodiment is substantially the same asthat for the first embodiment shown in FIGS. 2(A)-(E) up to the stepshown in FIG. 2(D).

First as shown in FIG. 2(A), there is prepared the SOS substrateincluding the sapphire substrate 101 and the silicon substrate 102, andthen the AlN layer 103 is grown on the silicon substrate 102 of the SOSsubstrate to the 200 nm thickness by the MOVPE method usingtrimethylaluminum and ammonia as the raw material gases, as shown inFIG. 2(B). This is followed by the growing of the GaN layer 100 on theAlN layer 103 to a thickness of 220 μm by the HVPE method using galliumchloride and ammonia as the raw material gases, as shown in FIG. 2(C).At this time, the GaN layer 100 has a surface which is a (0001) Gaplane. Thereafter the silicon substrate 102 is removed as shown in FIG.2(D), whereby the sapphire substrate 101 is separated from the GaN layer100 forming the nitride semiconductor substrate, i.e. the GaN substrate100.

Next, as shown in FIG. 10(A), the AlN layer 103 is removed by polishing,while the back surface of the GaN substrate 100 is made specular by apolishing using an abrasive with an extremely fine grain diameter.

Then, as shown in FIG. 10(B), the plurality of SiO₂ grains 106 a aredeposited on the GaN substrate 100 by an RF (radio frequency) sputteringmethod using argon gas. In the third embodiment, the gab pressure wasset at 0.2 Pa and the RF output was set at 200 W as conditions for thedeposition of the SiO₂ grains 106 a. The deposition conditions of theSiO₂ grains 106 a differ depending on the sputtering apparatus used; itis preferable, however, to employ such conditions that the SiO₂ grainsdeposited have as much grain diameter as possible, because thediscontinuous deposition of the SiO₂ grains is difficult under thecondition where an SiO₂ layer is uniformly deposited. In the case of RFsputtering, it is generally preferable to increase the diameter of thespattered grain by increasing the RF output. In this case, however, thegas pressure should preferably be lowered, for an increased RF outputtends to destabilize discharge.

An important point in the third embodiment is to terminate thedeposition of the SiO₂ grains 106 a in the step of FIG. 10(B) before theGaN substrate 100 is completely covered by the SiO₂ grains 106 a. Thetiming of termination of the deposition of the SiO₂ grains 106 adetermines the arrangement of the SiO₂ grains 106 a. For example, if thedeposition of the SiO₂ grains 106 a is stopped immediately before theGaN substrate 100 is completely covered by the SiO₂ grains 106 a, therearises a condition where openings are present at arbitrary points in theSiO₂ layer covering the GaN substrate 100. If the deposition of the SiO₂grains 106 a is stopped before the covering of the GaN substrate 100 bythe SiO₂ grains 106 a progresses, a plurality of scattered islands ofSiO₂ grains 106 a arise on the GaN substrate 100. In the thirdembodiment, the arrangement of the SiO₂ grains 106 a is not particularlylimited; however, in the case of making devices in which electrodes areprovided on either side of the GaN substrate 100 and current is flown inthe thickness direction, it is naturally preferable to reduce the areacovered on the GaN substrate 100 by the SiO₂ grains 106 a as much aspossible.

Next, the GaN substrate 100 provided with the SiO₂ grains 106 a isintroduced into the GaN crystal growing apparatus such as the MOVPEapparatus, where, as shown in FIG. 10(C), a GaN layer 107 is grown onthe GaN substrate 100 such that the spaces between the SiO₂ grains 106 aare filled. Specifically, in the MOVPE apparatus, the GaN layer 107 isgrown at a temperature of 1000° C. by using trimethylgallium and ammoniaas raw material gases, together with hydrogen as a carrier gas. Thegrowth conditions for the GaN layer 107 differ depending on the crystalgrowing method used. However, in the present embodiment where the MOVPEmethod is used, the crystal growing temperature is set at 900° C. ormore and the group V/group III raw-material supply ratio (ratio of thesupply flow rate of ammonia per minute to the supply flow rate oftrimethylgallium per minute) is set at 1000 or more. This way, themigration of the group-III raw material (Ga) can be activated, wherebythe GaN layer 107 can be grown in such a manner as to fill the spacesbetween the SiO₂ grains 106 a. While the surface of the GaN layer 107can be flattened if the GaN layer 107 is grown to a thickness of theorder of 10 μm, the crystal growth of the GaN layer 107 is ended beforea completely flat surface of the GaN layer 107 is obtained in the thirdembodiment.

Thereafter, as shown in FIG. 10(D), SiO₂ grains 106 a are newlydeposited on the GaN layer 107 (including the already deposited SiO₂grains 106 a) by an RF sputtering method. The GaN substrate 100 is thenre-introduced into the GaN crystal growing apparatus, where a crystalgrowth of a GaN layer 107 occurs. In the third embodiment, thedeposition of the SiO₂ grains 106 a and the crystal growth of the GaNlayer 107 are repeated until the thickness of the GaN layer 107 reachesabout 30 μm. Specifically, the RF sputtering for the deposition of theSiO₂ grains 106 a is effected six times in total, in order to form alight scattering portion 106 comprising the SiO₂ grains 106 a embeddedin the GaN layer 107, as shown in FIG. 10(E). Then, the GaN layer 107 isfurther crystal-grown to a thickness of about 50 μm by the HVPE methodusing gallium chloride and ammonia as raw material gases, therebycompleting the semiconductor substrate according to the third embodimentas shown in FIG. 9. In FIG. 10(E), the GaN layer 107 is not shown,because the GaN layer 107 is eventually integrated with the GaNsubstrate 100, which is formed from the same material.

In the third embodiment, the crystal growth of the GaN layer 107 wasstopped before the surface of the GaN layer 107 was made completely flatby the filling of the spaces between the SiO₂ grains 106 by the GaNlayer 107. If the spaces between the SiO₂ grains 106 a are filled bycrystal-growing the GaN layer 107 such that the surface of the GaN layer107 is completely flat, the SiO₂ grains 106 a are distributed in alaminar manner.

The present inventors formed a line-and-space resist pattern, similar tothe one used in the first embodiment, on the semiconductor substrateaccording to the third embodiment via a processed film by exposure usingg-line. When the processed film was etched using the resist pattern asthe mask, it was found that the size of the patterned processed film canalmost completely fall within a predetermined range by setting thetransmittance of the light scattering portion 106 at 80% or less. Thisis because the light incident on the substrate surface passes throughthe light scattering portion 106 twice before it is reflected by thesubstrate back surface and return to the substrate surface. Namely, eventhough the transmittance of the light scattering portion 106 is 80% andthe back surface reflectance on the specular plane of the substrate backsurface is about 20%, the net back-surface reflectance is 20%×80%×80%13%, thus allowing the satisfactory pattern formation to occur.

Thus, in accordance with the third embodiment, the intensity of thelight incident on the substrate surface and then reflected by thesubstrate back surface can be reduced by the light scattering portion106 formed inside the GaN substrate 100. This makes it possible to avoidthe problem of exposing the regions of the resist film other than thepredetermined exposure regions by the exposing beam of light incident onthe substrate surface and reflected by the substrate back surfacer inthe photolithography step for the manufacture of the semiconductordevice using the GaN substrate 10. Accordingly, the pattern accuracy inthe photolithography step can be improved and therefore themanufacturing yield of the nitride semiconductor device can beincreased.

In accordance with the third embodiment, after forming the SiO₂ grains106 a of a different material from that of the GaN substrate 100partially on the GaN substrate 100, a GaN layer (GaN layer 107) is newlycrystal-grown on the GaN substrate 100 including the SiO₂ grains 106 asuch that the GaN layer becomes part of the substrate. Accordingly,conveyance of any defects and the like present in the original GaNsubstrate 100 to the GaN layer can be prevented by the SiO₂ grains 106a. Thus, a satisfactory crystallinity of the GaN layer 107 can beensured and therefore a satisfactory crystallinity of the nitridesemiconductor substrate having the light scattering portion can beensured.

While in the third embodiment the deposition of the SiO₂ grains 106 aand the crystal-growth of the GaN layer 107 were each repeated aplurality of times, they may be each repeated at least one as long asthe light scattering portion 106 has a transmittance of 80% or less withrespect to the exposing beam of light such as g- or i-line. Since thetransmittance of the light scattering portion varies in a complex mannerdepending on the shape and density or the like of the SiO₂ grains 106 a,it is difficult to theoretically determine the shape and density or thelike of the SiO₂ grains 106 a so that the light scattering portion 106has the desired transmittance. Accordingly, it is preferable toexperimentally determine, for example, such a number of deposition ofthe SiO₂ grains 106 a or such thickness of the region where the SiO₂grains 106 a are embedded that the desired transmittance of the lightscattering portion 106 can be obtained.

In the third embodiment, the SiO₂ grains 106 naturally formed by the RFsputtering method were used as the light scattering portion 106.However, the method of forming the light scattering portion is notparticularly limited. For example, after forming the SiO₂ layer on theentire surface of the GaN substrate, a mask pattern can be partiallyformed on the SiO₂ layer. The SiO₂ layer can be etched using the maskpattern, thereby removing the portions of the SiO₂ layer not covered bythe mask pattern and forming the light scattering portion from thepatterned SiO₂ layer, when the mask pattern is removed. When using theSiO₂ grains as the light scattering portion, the diameter of each SiO₂grain should preferably be about 1/10 or more of the wavelength of theexposing beam of light. When using the patterned SiO₂ layer as the lightscattering portion, the width of the SiO₂ layer along a directionparallel to the substrate surface, or the thickness of the SiO₂ layer,should preferably be about 1/10 or more of the wavelength of theexposing beam of light. Regardless of whether the SiO₂ grains or thepatterned SiO₂ layer is used as the light scattering portion, thethickness of the light scattering portion should preferably be 1/10 ormore of the wavelength of the exposing beam of light.

While in the third embodiment SiO₂ was used as the material for thelight scattering portion, the material for the light scattering portionis not particularly limited as long as it is capable of having the GaNlayer crystal-grown thereon and has a different index of refraction thanthe of GaN. For example, Si, SiN or Al₂O₃ may be used instead of SiO₂.The light scattering portion does not have to be grains or layer madefrom single material. The grains or layers may be made from a pluralityof different materials that are capable of having the GaN layercrystal-grown thereon and having a different index of refraction thanthat of GaN, and such grains or layer may be used in combination orstacked. Alternatively, the grains or layers made from at least onematerial as mentioned above may be used in combination with air pores.

Furthermore, in the third embodiment, the type of the exposing beam oflight used in the photolithography is not particularly limited. However,the pattern accuracy can be greatly improved over the prior art by usinglight with certain wavelengths that can propagate through the GaNsubstrate 100 without being absorbed, such as g- or i-line.

In the third embodiment, the resist film used in the photolithographystep may be either positive or negative type.

While in the third embodiment GaN was used as the material for thenitride semiconductor substrate, this is merely exemplary and othergroup III nitride semiconductors such as GaN, InN, AlN or their mixedcrystals may be employed. In this case, the substrate may include othermaterials as long as any of those group III nitride semiconductorsconstitutes the main component of the substrate.

Embodiment 4

A semiconductor substrate according to a fourth embodiment of thepresent invention, a method of manufacturing the semiconductorsubstrate, and a pattern forming method for the manufacture of asemiconductor device using the semiconductor substrate will be describedby referring to the drawings.

FIG. 11 shows a cross-section of the semiconductor substrate accordingto the fourth embodiment.

As shown FIG. 11, the semiconductor substrate according to the fourthembodiment of the invention comprises a GaN layer 100 with a thicknessof 200 μm on the substrate surface side and an Al_(0.1)Ga_(0.9)N layer108 with a thickness of 15 μm on the substrate back surface side. Theback surface of the GaN layer 100, i.e., the interface between the GaNlayer 100 and the Al_(0.1)Ga_(0.9)N layer 108, is coarsely formed withheight irregularity 10 a. Thus the semiconductor substrate according tothe fourth embodiment comprises the semiconductor substrate (GaNsubstrate 100) with the height irregularity 100 a formed on its backsurface according to the first embodiment shown in FIG. 1, wherein theAl_(0.1)Ga_(0.9)N layer is formed on the height irregularity 100 a. Theheight irregularity 100 a should preferably have a height difference ofthe order of λ/10 or more, where λ is the wavelength of the exposingbeam of light used in the photolithography step for the manufacture ofthe semiconductor device using the semiconductor substrate according tothe present embodiment.

The method of manufacturing the semiconductor substrate according to thefourth embodiment shown in FIG. 11 is as follows. This method issubstantially the same as that for the semiconductor substrate accordingto the first embodiment shown in FIG. 2(A)-(E) up to the step shown inFIG. 2(E).

Specifically, there is first prepared the SOS substrate including thesapphire substrate 101 with the 300 μm thickness and the siliconsubstrate 102 with a thickness of 80 μm, as shown in FIG. 2(A). Then,the AlN layer 103 is grown on the silicon substrate 102 of the SOSsubstrate to the 200 nm thickness at temperature 1000° C. by the MOVPEmethod using trimethylaluminum and ammonia as the raw material gases, asshown in FIG. 2(B). This is followed by the growing of the GaN layer 100with the 250 μm thickness on the AlN layer 103 by the HVPE method usinggallium chloride and ammonia as the raw material gases, as shown in FIG.2(C). Thereafter the silicon substrate 102 is removed by a treatmentusing the liquid mixture of hydrofluoric acid and nitric acid, wherebythe sapphire substrate 101 is separated from the GaN layer 100 formingthe nitride semiconductor substrate, i.e., the GaN substrate 100, asshown in FIG. 2(D). Then, as shown in FIG. 2(E), the back surface of theGaN substrate 100 is polished by an abrasive with a grain diameter of10-50 μm, for example, down to a final thickness of the order of 200 μm,thereby removing the AlN layer 103 and forming the height irregularity100 a on the back surface of the GaN substrate 100. Thus the backsurface of the GaN substrate 100 is made coarse and there is obtainedthe GaN substrate 100 shown in FIG. 1.

Next, the Al_(0.1)Ga_(0.9)N layer 108 is grown on the back surface ofthe GaN substrate 100 where the height irregularity 100 a is formed, toa thickness of 15 μm at temperature 1000° C., by the MOVPE method usingtrimethylaluminum, trimethylgallium and ammonia as the raw materialgases. As a result, the semiconductor substrate according to the fourthembodiment shown in FIG. 11 is obtained. The height irregularity 100 aon the back surface of the GaN substrate 100 is filled by theAl_(0.1)Ga_(0.9)N layer 108, thereby flattening the back surface (whichmay hereunder be referred to simply as the substrate back surface) ofthe GaN substrate 100 including the Al_(0.1)Ga_(0.9)N layer 108.

The substrate back surface does not necessarily require the flattening.However, in the fourth embodiment, the Al_(0.1)Ga_(0.9)N layer 108 iscrystal-grown under the condition of flattening the substrate backsurface in order to determine the light scattering effect of the heightirregularity 100 a (see the first embodiment) in a state where thesubstrate back surface does not produce any scattering of light. Theconditions for crystal-growing the Al_(0.1)Ga_(0.9)N layer 108 such thatthe flattening of the substrate back surface is achieved differdepending on the crystal-growth method used. However, when the MOVPEmethod is used, as in the present embodiment, the migration of the groupIII material (Ga or Al) can be activated by setting the crystal-growthtemperature at 900° C. or more and setting the group V/group IIIraw-material supply ratio (ratio of the supply flow rate of ammonia perminute to the supply flow rate of trimethylaluminum or trimethylgalliumper minute) at 1000 or more. Thus, the height irregularity 100 a can befilled by the Al_(0.1)Ga_(0.9)N layer 108 to flatten the substrate backsurface.

The present inventors formed a line-and-space resist pattern similar tothe one used in the first embodiment on the semiconductor substrateaccording to the fourth embodiment via a processed film by exposureusing g-line. When the processed film was etched using the resistpattern as a mask, it was found that the size of the patterned processedfilm substantially fell within a predetermined range by setting thetransmission of the height irregularity 100 a, i.e., the lightscattering portion, with respect to the exposing beam of light at 80% orless. This is because the light incident on the substrate surface andthen reflected by the substrate back surface back to the substratesurface passes through the light scattering portion twice. Namely, evenwhen the transmission of the light scattering portion is 80% and theback surface reflectance of the substrate back which is specular is ofthe order of 20%, the net back surface reflectance is 20%×80%×80%≈13%,making it possible to achieve a satisfactory pattern formation.

Thus, in accordance with the fourth embodiment, the height irregularity100 a formed at the interface between the GaN layer 100 and theAl_(0.1)Ga_(0.9)N layer 108 functions as the light scattering portion,thereby reducing the intensity of the reflected beam of light caused bythe incident beam of light incident on the substrate surface andreflected by the substrate back surface. Accordingly, the problem ofexposing the regions of the resist film other than the predeterminedexposure regions by the exposing beam of light incident on the substratesurface and reflected by the substrate back surface can be avoided. As aresult, the pattern accuracy in the photolithography step can beimproved and therefore the manufacturing yield of the nitridesemiconductor device can be improved.

In accordance with the fourth embodiment, the back surface of the GaNsubstrate 100 made coarse by the height irregularity 100 a can beflattened by the Al_(0.1)Ga_(0.9)N layer 108, thereby simplifying themanufacturing process of the semiconductor device.

Furthermore, in accordance with the fourth embodiment, theAl_(0.1)Ga_(0.9)N layer 108 as the group III nitride semiconductor layeris crystal-grown on the back surface of the GaN substrate 100.Accordingly, a satisfactory crystallinity can be obtained in theAl_(0.1)Ga_(0.9)N layer 108 formed on the convex portion of the heightirregularity 100 a, so that a satisfactory crystallinity can be obtainedin the nitride semiconductor substrate having the light scatteringportion.

While in the fourth embodiment the Al_(0.1)Ga_(0.9)N layer 108 wasformed on the back surface of the GaN substrate 100 with the heightirregularity 100 a, the light scattering effect of the heightirregularity 100 a can be also realized by forming a layer of othermaterial (the layer may be other than a nitride semiconductor layer)with a different index of refraction than that of the GaN substrate 100.In this case, by providing a material layer on the back surface of theGaN substrate 100 with the height irregularity 100 a that functions asan anti-reflection film (see the second embodiment or its variation),the intensity of the reflected beam of light can be further reduced. Inthe fourth embodiment, the thickness of the Al_(0.1)Ga_(0.9)N layer 108is made very thick (15 μm) as compared with that of the wavelength ofthe exposing beam of light such as g- or i-line, in order to fill theheight irregularity 100 a. As a result, the Al_(0.1)Ga_(0.9)N layer 108hardly functions as an anti-reflection film. Accordingly, in the fourthembodiment, the reflected beam of light consists of the light reflectedby the interface (with a reflectance of about 21%) between theAl_(0.1)Ga_(0.9)N layer 108 and air, and the light reflected by theinterface (whose reflectance varies depending on the shape and densityof the height irregularity 100 a) between the GaN substrate 100 and theAl_(0.1)Ga_(0.9)N layer 108. Thus, in the fourth embodiment, there is apossibility that the light scattered by the interface between the GaNsubstrate 100 and the Al_(0.1)Ga_(0.9)N layer 108, i.e., the heightirregularity 100 a, is reflected by the interface between theAl_(0.1)Ga_(0.9)N layer 108 and air to be eventually returned to thesurface of the GaN substrate 100. As mentioned above, when aline-and-space (with a line and space width of 2 μm) resist patternsimilar to the one used in the first embodiment is formed by exposureusing g-line, a satisfactory pattern formation was achieved. This ispresumably because of the fact that about 50% of the photomask wasoccupied by the opening. In other words, in the fourth embodiment, asthe area occupied by the opening in the photomask increases, it becomesincreasingly difficult to ignore the effect of the light exposing theresist film from below after being scattered by the height irregularity100 a and then reflected by the interface between the Al_(0.1)Ga_(0.9)Nlayer 108 and air.

Further, in the fourth embodiment, the type of the exposing beam oflight used in the photolithography step is not particularly limited.However, a great improvement in the pattern accuracy can be obtainedover the prior art by using light with certain wavelengths that canpropagate through the GaN substrate 100 without being absorbed, such asg- or i-line, as the exposing beam of light.

In the fourth embodiment, the resist film used in the photolithographystep may be either positive or negative type.

In the fourth embodiment, while GaN was used as the material for thenitride semiconductor substrate, this is merely exemplary and a groupIII nitride semiconductor of GaN, InN, AlN or their mixed crystals maybe used. In this case, the substrate may include other materials as longas those group III nitride semiconductor constitute a main component ofthe substrate.

Embodiment 5

Hereunder a semiconductor substrate according to a fifth embodiment ofthe present invention, a method of manufacturing the semiconductorsubstrate, and a pattern forming method for the manufacture of asemiconductor device using the semiconductor substrate will be describedby referring to the drawings.

FIG. 12 shows a cross sectional view of the semiconductor substrateaccording to the fifth embodiment.

As shown in FIG. 12, the semiconductor substrate according to the fifthembodiment comprises a group III nitride semiconductor layer,specifically a GaN layer 100. The GaN layer 100 (which may hereunder bereferred to as a GaN substrate 100) has a surface which is a (0001) Gaplane and a back surface which is a (0001) N plane. The thickness of theGaN substrate 100 is 300 μm, for example.

The fifth embodiment is characterized in that a plurality of Si layers109 a of a material with a larger absorption coefficient than that ofGaN with respect to an exposing beam of light such as g- or i-line, suchas Si, for example, are discontinuously embedded inside the GaNsubstrate 100 at regions with depths of the order of 80 μm and 75 μm.Those plurality of Si layers 109 a form a light absorbing portion (forabsorbing light incident on the substrate surface) 109. The width ofeach Si layer 109 a (along a direction parallel to the substratesurface) is about 1 μm, and there is about a 1 μm spacing providedbetween the neighboring Si layers 109 a. Thus the Si layers 109 a arearranged in a periodic stripe. Each Si layer 109 a has a height (in adirection perpendicular to the substrate surface) of about 10 nm. In theGaN substrate 100, the Si layer 109 a (a first layer) in the regionabout 80 μm below the surface and the Si layer 109 a (a second layer) inthe region 75 μm below the surface are staggered with respect to eachother by about 1 μm along a direction parallel to the substrate surface.

FIGS. 13(A)-(E) and FIGS. 14(A)-(C) show cross sectional viewsillustrating the method of manufacturing the semiconductor substrateaccording to the fifth embodiment of the invention. This method issubstantially identical to the one for the semiconductor substrateaccording to the first embodiment shown in FIGS. 2(A)-(E) up to the stepshown in FIG. 2(D).

Specifically, there is first prepared the SOS substrate as shown in FIG.2(A), including the sapphire substrate 101 and the silicon substrate102. Then, as shown in FIG. 2(B), the AlN layer 103 is grown on thesilicon substrate 102 of the SOS substrate to the thickness of 200 nm bythe MOVPE method using the trimethylaluminum and ammonia as the rawmaterial gases. Thereafter the GaN layer 100 is grown on the AlN layer103 to a thickness of 220 μm by the HVPE method using the galliumchloride and ammonia as the raw material gases, as shown in FIG. 2(C).In this case, the surface of the GaN layer 100 is a (0001) Ga plane.This is followed by the removal of only the silicon substrate 102, asshown in FIG. 2(D), whereby the sapphire substrate 101 is separated fromthe GaN layer 100 forming the nitride semiconductor substrate, i.e. theGaN substrate 100.

Next, as shown in FIG. 13(A), the AlN layer 103 is removed by polishingwhile the back surface of the GaN substrate 100 is made specular bypolishing by an abrasive with an extremely fine grain diameter.

Thereafter a first Si layer 110 is deposited on the GaN substrate 100 toa thickness of 100 nm by an RF sputtering as shown in FIG. 13(B). Then,a first resist pattern 111 is formed on the first Si layer 110 byphotolithography, the resist pattern having cyclically arrangedopenings, as shown in FIG. 13(C)

The first Si layer 110 is then wet-etched by using hydrogen fluorideusing the first resist pattern 111 as a mask, thereby patterning thefirst Si layer 110 in a cyclic stripe shape, as shown in FIG. 13(D). Thefirst resist pattern 111 is thereafter removed by means of an organicsolvent as shown in FIG. 13(E).

The GaN substrate 100 is then introduced into a GaN crystal growingapparatus such as an MOVPE apparatus, where a first GaN layer 112 isgrown on the GaN substrate 100 such that the first Si layer 110 isburied, as shown in FIG. 14(A). Specifically, in the MOVPE apparatus,the first GaN layer 112 is grown at a temperature of 1000° C. usingtrimethylgallium and ammonia as raw material gases together withhydrogen as a carrier gas. The growth conditions for the first GaN layer112 differ depending on the crystal-growth method used. In the case ofthe present embodiment, where the MOVPE method is used, thecrystal-growth temperature is set at 900° C. or more and the groupV/group III raw-material supply ratio (ratio of the supply flow rate ofammonia per minute to the supply flow rate of trimethylgallium perminute) at 1000 or more. This way, the migration of the group IIImaterial (Ga) can be activated, making it possible to grow the first GaNlayer 112 in such a manner as to bury the first Si layer 110.

Next, as shown in FIG. 14(B3), a second Si layer 113 is deposited on thefirst GaN layer 112 to a thickness of boom by an RF sputtering. This isfollowed by the formation of a second resist pattern not shown) on thesecond Si layer 113 in the same manner as in the step of FIG. 13(C).Thereafter the second Si layer 113 is etched using the second resistpattern as a mask in the same manner as in the step of FIG. 13(D),whereby the second Si layer 113 is patterned in a cyclic stripe shape,as shown in FIG. 14(B).

The second resist pattern is then removed. It should be noted that thefirst Si layer 110 and the second Si layer 113 are staggered with eachother with a predetermined distance when patterned.

The GaN substrate 100 is then re-introduced into the GaN crystal-growthapparatus such as the MOVPE apparatus, where a second GaN layer 114 isgrown on the first GaN layer 112 such that the second Si layer 113 isburied, as shown in FIG. 14(C). In this case, the growth conditions forthe second GaN layer 114 are the same as those for the first GaN layer112. The first GaN layer 112 and the second GaN layer 114 are eventuallyintegrated with the GaN substrate 100 formed from the same material.Thus, there is formed inside the GaN substrate 100 including the firstGaN layer 112 and the second GaN layer 114 a light absorbing portion 109including the patterned first Si layer 110 and second Si layer 113(i.e., a plurality of Si layers 109 a shown in FIG. 12).

The present inventors formed a line-and-space resist pattern similar tothe one used in the first embodiment on the semiconductor substrate viaa processed film by exposure using g-line. When the processed film wasetched using the resist pattern as a mask, it was found that the size ofthe patterned processed film can be maintained substantially completelywithin a predetermined range by setting the transmittance of the lightabsorbing portion 109 below 80%. This is because of the fact that thelight incident on the substrate surface and then reflected by thesubstrate back surface back to the substrate surface passes through thelight absorbing portion 109 twice. Namely, even when the transmittanceof the light absorbing portion 109 is 80% and the back surfacereflectance of the specular substrate back surface is about 20%, the netback surface reflectance is 20%×80%×80%≈13%, which makes it possible toachieve a satisfactory pattern formation.

Thus, in accordance with the fifth embodiment, since the light absorbingportion 109 is formed inside the GaN substrate 100, the light incidenton the substrate surface and then reflected by the substrate backsurface can be reduced. This prevents the problem of exposing theregions of the resist film other than the predetermined exposure regionsby the exposing beam of light incident on the substrate surface and thenreflected by the substrate back surface, in the photolithography stepfor the manufacture of the semiconductor device using the GaN substrate100. Thus the pattern accuracy in the photolithography step can beincreased and therefore the manufacturing yield of the nitridesemiconductor device can be increased.

In accordance with the fifth embodiment, the GaN layers (the first GaNlayer 112 and the second GaN layer 114) are newly crystal-grown on theGaN substrate 100 including the Si layer 109 a in such a manner as to bepart of the substrate only after the partial formation of the Si layers109 a on the GaN substrate 100 a, the Si layers 109 a and the GaNsubstrate 100 a made from different materials. Thus the Si layers 109 acan prevent any defects and the like present in the original GaNsubstrate 100 from being conveyed to the newly formed GaN layers.Accordingly, a satisfactory crystallinity can be obtained in the newlygrown GaN layers and therefore a satisfactory crystallinity can also beobtained in the nitride semiconductor substrate having the lightabsorbing portion.

Further, in accordance with the fifth embodiment, since the Si layers109 a forming the light absorbing portion 109 are distributednon-uniformly along a direction parallel to the substrate surface, notonly does the light absorbing portion 109 absorb light but also scatterlight, so that the intensity of the reflected beam of light can befurther reduced.

In the fifth embodiment, since the Si layers 109 a forming the lightabsorbing portion 109 are electrically conductive, the provision of thelight absorbing portion does not cause any reduction in the resistivityof the nitride semiconductor substrate.

While in the fifth embodiment Si was used as the material for the lightabsorbing portion, this is only exemplary and other materials may beused as long as they are capable of having a GaN layer crystal-grownthereon and have a larger light absorption coefficient than GaN. Itshould be noted, however, that the material for the light absorbingportion should preferably be electrically conductive. Such materialincludes W (tungsten) as well as Si. Though tungsten is a metal,tungsten deposited by CVD and the like has little luster and yet has alight absorbing property, making it an ideal material for the lightabsorbing portion. The light absorbing portion does not necessarilycomprise grains or layers of single material; grains or layers ofdifferent materials that are capable of having a GaN layer crystal-grownthereon and have larger light absorption coefficients than GaN may beused in combination or laminated. Alternatively, grains or layers of atleast one material as mentioned above may be used in combination withair pores.

The absorption coefficient of the material Si of the light absorbingportion in the present embodiment greatly varies depending on the methodof depositing the Si layer, the film substance or the containedimpurities and the like. The absorption coefficient reaches as much asabout 10⁵/cm or more if the thickness of the Si layer is in the range of300-500 nm. If the thickness of the Si layer is at least 100 nm, theintensity of light passing though the Si layer is reduced to at least1/e (e: base of natural logarithm), so that the transmission of the Silayer can be made 80% or less regardless of the deposition method, andthe Si layer thus functions satisfactorily as the light absorbingportion. On the other hand if the thickness of the Si layer is less than100 n, particularly less than 50 nm, the transmission of the Si layermay become 80% or more depending on the film substance, in which casethe Si layer may not function satisfactorily as the light absorbingportion.

While in the fifth embodiment, the type of the exposing beam of lightused in the photolithography step is not particularly limited, thepattern accuracy can be significantly improved over, the prior art byusing light with certain wavelengths that can propagate through the GaNsubstrate 100 without being absorbed, such as g- or i-line, as theexposing beam of light.

In the fifth embodiment, the resist film used in the photolithographystep may be either positive or negative type.

While in the fifth embodiment, GaN was used as the material for thenitride semiconductor substrate, other group III nitride semiconductorssuch as GaN, InN, AlN or their mixed crystals may be used. In this case,the substrate may include other materials as long as those group IIInitride semiconductors constitute a main component of the substrate.

Embodiment 6

Hereunder a semiconductor substrate according to a sixth embodiment ofthe invention, a method of manufacturing the semiconductor substrate,and a pattern forming method for the manufacture of a semiconductordevice using the semiconductor substrate will be described by referringto the drawings.

FIG. 15 shows a cross sectional view of the semiconductor substrateaccording to the sixth embodiment.

As shown, the semiconductor substrate according to the sixth embodimentcomprises a group III nitride semiconductor layer, specifically a GaNlayer 200. The GaN layer 200 (which may hereunder referred to as a GaNsubstrate 200) has a surface which is a (0001) Ga plane and a backsurface which is a (0001) N plane. The thickness of the GaN substrate200 is 300 μm, for example.

The sixth embodiment is characterized in that impurities such as As(arsenic) that produce a level for absorbing an exposing beam of lightsuch as g- or i-line are introduced into the GaN substrate 200 in aregion stretching 150 μm from the back surface, the impurities forming alight absorbing portion 201.

FIGS. 16(A)-(F) are cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor substrateaccording to the sixth embodiment shown in FIG. 15.

As shown in FIG. 16(A), there is first prepared an SOS substrateincluding a sapphire substrate 202 with a thickness of 300 μm and asilicon substrate 203 with a thickness of 80 μm.

Next, an AlN layer 204 is grown on the silicon substrate 203 of the SOSsubstrate at a temperature of 1000° C. to a thickness of a 200 nm by anMOVPE method using trimethylgallium and ammonia as raw material gases,as shown in FIG. 16(B).

Thereafter a GaN layer 201 is grown on the AlN layer 204 at atemperature of 1000° C. to a thickness of a 150 μm by an HVPE methodusing gallium chloride obtained by reacting HCl gas with Ga at atemperature of 800° C. and ammonia as raw material gases, as shown inFIG. 16(C). By introducing a gas having As such as arsine as aconstituent element into the crystal-growth apparatus at a supply flowrate of e.g. 0.1% of the ammonia supply flow rate, the light absorbingportion 201 can be formed in the GaN layer 201 where As has beenintroduced. Instead of introducing the gas such as arsine, As can alsobe introduced into the GaN layer by placing a GaAs crystal near theregion where the HCl gas and Ga are reacted in the crystal-growthapparatus, whereby the light absorbing portion 201 can be formed.

Then, only the supply of arsine is stopped while allowing the crystalgrowth by the HVPE method, so that a GaN layer 200 is grown on the lightabsorbing portion 201 to a thickness of 150 μm at a temperature of 1000°C., as shown in FIG. 16(D).

This is followed by a treatment using a liquid mixture of hydrogenfluoride to remove only the silicon substrate 203, as shown in FIG.16(E), whereby the sapphire substrate 202 is separated from the GaNsubstrate 200 including the light absorbing portion 201 forming thenitride semiconductor substrate. On the back surface of the GaNsubstrate 200 is formed the AlN layer 204.

The AlN layer 204 is then removed by polishing using an abrasive withextremely fine grain diameter while making the back surface of the GaNsubstrate 200 specular, as shown in FIG. 16(F).

The present inventors formed a line-and-space resist pattern similar tothe one used in the first embodiment on the semiconductor substrateaccording to the sixth embodiment via a processed film by exposure usingg-line. When the processed film was etched using the resist pattern as amask, it was found that the size of the patterned processed film can bemade to fall almost completely within a predetermined range by settingthe transmission of the light absorbing portion 201 at 80% or less. Thisis because the light incident on the substrate surface passes throughthe light absorbing portion 201 twice before it is reflected by thesubstrate back surface and return to the substrate surface. Namely, eventhough the transmittance of the light absorbing portion 201 is 80% andthe back surface reflectance on the specular plane of the substrate backsurface is about 20%, the net back-surface reflectance is 20%×80%×80%13%, thus allowing the satisfactory pattern formation to occur.

Thus, in accordance with the sixth embodiment, since the light absorbingportion 201 is formed in a region on the back surface side of the GaNsubstrate 200, the intensity of light incident on the substrate surfaceand then reflected by the substrate back surface can be reduced. Thisprevents the problem of exposing the regions of the resist film otherthan predetermined exposure regions by the exposing beam of lightincident on the substrate surface and then reflected by the substrateback surface in the photolithography step for the manufacture of thesemiconductor device using the GaN substrate 200. Accordingly, thepattern accuracy in the photolithography step can be improved andtherefore the manufacturing yield of the nitride semiconductor devicecan be increased.

Furthermore, in accordance with the sixth embodiment, the lightabsorbing portion 201 is formed by implanting impurities into the GaNlayer forming the GaN substrate 200, a lowering of the crystallinity ofthe nitride semiconductor substrate with the light absorbing portion canbe prevented.

Hereunder, necessary conditions for lowering the transmission of thelight absorbing portion 201 to 80% or below will be explained.

Generally, introduction of impurities of certain kind into asemiconductor produces a light absorbing property. Now, let z be adirection perpendicular to a main plane of the semiconductor substrate,α(z) the light absorption coefficient at position z in the semiconductorsubstrate, and I(z) the light intensity at position z. Based on arelationship between the amount of attenuation of light with lightintensity I(z) and the light absorption coefficient, the followingequation (1) is satisfied. $\begin{matrix}{\frac{\mathbb{d}{I(z)}}{\mathbb{d}z} = {{- {\alpha(z)}} \times {I(z)}}} & \left( {{equation}\quad 1} \right)\end{matrix}$

Assume now that the region in which the impurities are introduced in thesemiconductor substrate, i.e. the light absorbing portion, isdistributed from position z1 to position z2. Let z0 be the thickness ofthe light absorbing portion (z2−z1), I0 the light intensity at the pointof incidence on the light absorbing portion, and I the light intensityat the point of exit from the light absorbing portion, and the followingequation (2) is satisfied. $\begin{matrix}{{\log\left( \frac{I}{\quad{Io}} \right)} = {{\int_{z\quad 1}^{z\quad 2}{{- {\alpha(z)}}{\mathbb{d}z}}} = {z\quad 0{\int_{z\quad 1}^{z\quad 2}{\frac{- {\alpha(z)}}{z\quad 0}\quad{\mathbb{d}z}}}}}} & \left( {{equation}\quad 2} \right)\end{matrix}$

Since the equation $\begin{matrix}{\int_{z\quad 1}^{z\quad 2}{\frac{\alpha(z)}{z\quad 0}\quad{\mathbb{d}z}}} & \left( {{equation}\quad 3} \right)\end{matrix}$gives an average of the light absorption coefficient α(z) from positionz1 to z2, substituting α for this equation, the following equation (4)is satisfied. $\begin{matrix}{\frac{I}{Io} = {\exp\left( {{- \alpha} \times z\quad 0} \right)}} & \left( {{equation}\quad 4} \right)\end{matrix}$

Here, based on the condition that the transmittance I/I0 is 80% or less,finding the range of the thickness z0 of the light absorbing portionusing equation (4) gives $\begin{matrix}{{z\quad 0} \geqq \frac{0.223}{\alpha}} & \left( {{equation}\quad 5} \right)\end{matrix}$

Thus, 0.223/α is the minimum thickness of the light absorbing portion201 necessary for making the transmittance of the light absorbingportion 201 80% or less.

On the other hand, with regard to the impurity density in the lightabsorbing portion 201, while that a higher impurity density ispreferable because that will result in a higher α and therefore permit areduction in the thickness of the light absorbing portion 201, i.e., theimpurity layer, too high an impurity density will cause a displacementin the lattice constant of the crystals in the substrate, resulting in adeteriorated crystallinity of the substrate.

Accordingly, in the case of As as the impurity for forming the lightabsorbing portion 201, its impurity density should preferably lie withina range from the order of 1×10¹³ cm⁻³ to 1×10²⁰ cm⁻³.

While in the sixth embodiment, As was used as the impurity for formingthe light absorbing portion 201, any other impurity may be used as longas it produces a level that absorbs the exposing beam of light whenintroduced into the GaN layer. For example, C (carbon), O (oxygen), Si,S (sulfur), Cl (chlorine) or P (phosphorus) may be used instead of As.Even when using C, O, Si, S, Cl or P as the impurity for forming thelight absorbing portion 201, too, the impurity density should preferablylie within the range from the order of 1×10³ cm⁻³ to 1×10²⁰ cm⁻³. Whenintroducing C in the GaN layer, a C-containing gas such as CH₄ may beused during the crystal growth of the GaN layer. When introducing O intothe GaN layer, an O-containing gas such as NO₂ may be used during thecrystal growth of the GaN layer. When Si is introduced into the GaNlayer, a Si-containing gas such as SiH₄ may be used during the crystalgrowth of the GaN layer. When S is introduced into the GaN layer, aS-containing gas such as SF6 may be used during the crystal growth ofthe GaN layer. When Cl is introduced in to the GaN layer, e.g. the groupV/group III raw-material supply ratio (ratio of the supply flow rate ofammonia per minute to the supply flow rate of gallium chloride perminute) may be set at 100 or less, thereby facilitating the entry of Clinto the nitride site of GaN. When P is introduced into the GaN layer,e.g. phosphine may be mixed with ammonia during the crystal growth ofthe GaN layer. Further, by introducing C, O, Si, S, Cl, P or As asmentioned above to the GaN layer as impurities while properly selectingthe used gas, the occurrence of contamination and the like in thecrystal growth apparatus can be prevented.

While in the sixth embodiment the light absorbing portion 201 wasprovided in a region on the back surface side of the GaN substrate 200,the entire substrate may constitute the light absorbing portion byintroducing the impurity into the entire substrate, as long as thethickness z0 of the light absorbing portion satisfies equation (5).

While in the sixth embodiment the light absorbing portion 201 wasuniformly provided in a region on the back surface side of the GaNsubstrate 200, the light absorbing portion may instead be distributednon-uniformly along a direction parallel to the substrate surface. Inthis way, not only does the light absorbing portion absorbs light butalso it can scatter light, thereby further reducing the intensity of thereflected beam of light. In this case, by introducing the impurity onlyto an area below the predetermined exposure region of the resist film inthe GaN substrate 200, the following advantages can be obtained. Forexample, when a device is formed by covering an area near the activelayer on the substrate by a resist pattern, as when a ridge-type laserdevice is to be produced, by not introducing the impurity into an areabelow the active layer of the substrate so that there is less impuritybeing diffused near the active layer from the substrate, an increase ofthe operating current and the like due to light absorption by theimpurity can be prevented. With regard to the semiconductor layers otherthan the active layer on the substrate, as the pattern accuracy in thephotolithography step increases, the pattering yield can be increased.For doping the impurity into the GaN substrate such that the lightabsorbing portion is distributed non-uniformly, there are variousmethods including the one using an ion beam to ion-inject the impurityinto the GaN substrate, or the one based on a combination of a selectivegrowth and an embedded growth for growing the nitride semiconductorlayer for the formation of the substrate. In the latter method, regionswhere the impurity is not to be doped are covered by a mask and thenitride semiconductor layer is selectively grown by doping the impurity,and then the nitride semiconductor layer is embedded-grown withoutdoping the impurity.

Further, while in the sixth embodiment the GaN layer was provided withthe light absorbing property by the introduction of the impurity intothe GaN layer, the giving of the light absorbing property to the GaNlayer may be effected by forming a point defect in the GaN layer. Inthis case, the point defect can be formed in the GaN layer by implantinge.g. proton into the GaN layer.

The type of the exposing beam of light used in the photolithography stepin the sixth embodiment is not particularly limited. However, by usinglight with certain wavelength that can propagate through the GaNsubstrate 200 without being absorbed, such as g- or i-line, as theexposing beam of light, the pattern accuracy can be greatly improvedover the prior art.

The resist film used in the photolithography step in the sixthembodiment may be either positive or negative type.

While in the sixth embodiment GaN was used as the material for thenitride semiconductor substrate, this is merely exemplary and othergroup III nitride semiconductors such as GaN, InN, AlN or their mixedcrystals may be used. In this case, the substrate may include othermaterials as long as those group III nitride semiconductors constitute amain component of the substrate.

Embodiment 7

Hereunder a semiconductor substrate according to a seventh embodiment ofthe present invention, a method of manufacturing the semiconductorsubstrate, a semiconductor device using the semiconductor substrate anda method of manufacturing the semiconductor device will be described byreferring to the drawings.

FIG. 17 is a cross sectional view of the semiconductor substrateaccording to the seventh embodiment.

As shown in FIG. 17, the semiconductor substrate according to theseventh embodiment comprises a group III nitride semiconductor layer,specifically a GaN layer 300. The thickness of the GaN layer 300 (whichmay hereunder be referred to as a GaN substrate 300) is e.g. 250 μm, andeither side of the GaN layer is made specular.

The seventh embodiment is characterized in that a plurality of lightabsorbing portions (for absorbing light incident on the substratesurface) 301 are formed in a stripe shape by introducing an impuritysuch as As that produces a level absorbing an exposing beam of lightsuch as g- or i-line into a surface portion of the GaN substrate 300.Each light absorbing portion 301 has a width (along a direction parallelto the substrate surface) of the order of 310 μm. The light absorbingportions 301 are distanced from each other by about 10 μm.

FIGS. 18(A)-(E) shows cross sectional views illustrating the respectivesteps of the method of manufacturing the semiconductor substrateaccording to the seventh embodiment of FIG. 17.

First, as shown in FIG. 18(A), there is prepared a GaN substrate 300with a thickness of 300 μm having height irregularity 300 a formed onits back surface, the height irregularity 300 a having a heightdifference of the order of 1/10 the wavelength of the exposing beam oflight. The GaN substrate 300 may be produced by the method ofmanufacturing the semiconductor substrate according to the firstembodiment.

Then, as shown in FIG. 18(B), a plurality of hard masks 302 made of SiO₂are formed by photolithography on the surface of the GaN substrate 300.Here, the width of each hard mask 302 is 300 μm, and the distancebetween the neighboring hard masks 302 is 20 μm. In the photolithographystep for the formation of the hard masks 302, the height irregularity300 a on the back surface of the GaN substrate 300 functions as thelight scattering portion (see the first embodiment), so that the patternaccuracy of the hard masks 302 can be improved.

Thereafter As is introduced into the surface portion of the GaNsubstrate 300 by means of the hard masks 302, as shown in FIG. 18(C),whereby a plurality of light absorbing portions 301 are formed in astripe shape.

While the method of implanting As into the GaN substrate 300 is notparticularly limited, an example of the As implantation method in themethod of manufacturing the semiconductor substrate according to theseventh embodiment will hereinafter be described by referring to FIG.19. As shown in FIG. 19, after forming a GaAs layer 303 on the surfaceof the GaN substrate 300 provided with the hard masks 302, the GaNsubstrate 300 is placed on a susceptor 351 within a reaction pipe 350made from quartz and the like. Then, ammonia is supplied from a gasinlet 350 a of the reaction pipe 350, while ammonia near the susceptor351 is heated to a temperature of about 1000° C. by a heating means 352provided externally to the reaction pipe 350. As the As in the GaAslayer 303 diffuses into the surface portion of the GaN substrate 300,the light absorbing portion 301 is formed. Used ammonia is dischargedfrom a gas outlet 350 b of the reaction pipe 350. The susceptor 351 ismade from, for example, graphite. The heating means 352 may comprise atubular resistance-wire heater.

The As implantation in the atmosphere of ammonia as shown in FIG. 19 iscarried out so that the escape of nitrogen from the back surface (coarseplane) of the GaN substrate 300 can be prevented by nitrogen produced bythe decomposition of ammonia. Therefore, the As implantation may beeffected in an atmosphere of another gas containing nitrogen atoms,instead of ammonia.

Since the GaAs layer 303 formed on the GaN substrate 300 does not haveto be a single crystal, the GaAs layer 303 may be formed by sputtering.Alternatively, a compound layer containing an As layer or As may beformed, instead of the GaAs layer 303.

The temperature during the diffusion of As in the GaN substrate 300 maybe lower than 1000° C. However, in this case, since the diffusion rateof As decreases, it is necessary to diffuse As over a longer period oftime in order to obtain a desired distribution profile of the lightabsorbing portions 301, i.e., a desired As diffusion profile. It ispreferable, however, to set the diffusion temperature of As at 700° C.or more when As is to be diffused such that the transmittance of thelight absorbing portions 301 is sufficiently lowered within a practicaldiffusion time of the order of several hours to several tens of hours.

The width of each light absorbing portion 301 obtained by the Asimplantation step of FIG. 19 becomes 310 μm which is wider than thewidth of each hard mask 302 due to the diffusion of As. Accordingly, theintervals between the light absorbing portions 301 becomes 10%. Thethickness of each light absorbing portion 301 is about 5 μm.

The hard masks 302 are then removed by a wet etching using e.g. hydrogenfluoride as shown in FIG. 18(D), and thereafter the back surface of theGaN substrate 300 having the height irregularity 300 a is polished,thereby making the back surface specular, as shown in FIG. 18(E).

The resultant GaN substrate 300 with the light absorbing portions 301 isadvantageous in that when making a nitride semiconductor device usingthe GaN substrate 300, e.g. a high performance ridge-type laser devicewith a low operating current can be manufactured at high yields.

Hereunder, the method of manufacturing the semiconductor device usingthe semiconductor substrate according to the seventh embodiment,specifically a ridge-type laser device, will be described by referringto the FIGS. 20(A)-(D) and FIGS. 21(A)-(D).

As shown in FIG. 20(A), there is first prepared the GaN substrate 300having the light absorbing portions 301 (see FIG. 17). In the followingdescription, only a region sandwiched by a pair of light absorbingportions 301 in the GaN substrate 300 (i.e. a ridge-structure formedregion) and adjacent areas will be referred.

As shown in FIG. 20(B), there are sequentially formed on the GaNsubstrate 300 an n-type Al_(0.1)Ga_(0.9)N clad layer with a thickness of1 μm, a quantum-well active layer composed of an In_(0.2)Ga_(0.8)N welllayer with a thickness of 30 nm and an In_(0.02)Ga_(0.98)N barrierlayer, and a p-type Al_(0.1)Ga_(0.9)N clad layer 312 with a thickness of2 μm. The formation of those respective nitride semiconductor layer maybe effected by an MOVPE method, for example. When the respective nitridesemiconductor layers are formed by the MOVPE method at a temperature ofthe order of 1000° C., for example, the As in the light absorbingportions 301 is further diffused. As a result, as shown in FIG. 20(B),the distributed regions of the light absorbing portions 301 furtherexpand. Thus, in the vicinity of the interface between the GaN substrate300 and the n-type Al_(0.1)Ga_(0.9)N clad layer 310, for example, thedistance between the neighboring light absorbing portions 301 is of theorder of 2-3 μm, and that of the neighboring light absorbing portions301 in the vicinity of the quantum-well active layer 311 is of the orderof 5 μm.

Thereafter, a positive-type resist film 313 is formed on the p-typeAl_(0.1)Ga_(0.9)N clad layer 312 as shown in FIG. 20(C). This isfollowed by an irradiation of the resist film 313 with an exposing beamof light of g-line via a photomask 360 covering the ridge-structureformed region (between the neighboring light absorbing portions 301)with a width of about 3 μm, as shown in FIG. 20(D). Due to the lightabsorbing portions 301 in the regions not covered by the photomask 360in the GaN substrate 300 (including the respective nitride semiconductorlayers), the intensity of the reflected beam of light from the backsurface of the GaN substrate 300 is reduced during exposure.Accordingly, it becomes possible to prevent the problem of exposing theportions below the photomask 360 in the resist film 313 by the exposingbeam of light diffracted inside the photomask 360 near the photomask 360and then reflected by the back surface of the GaN substrate 300.

Then, as shown in FIG. 21(A), the resist film 313 is developed and theportions of the resist film 313 that were irradiated by the exposingbeam of light are removed, thereby forming a resist pattern 313A. Inthis case, since the unwanted exposure of the resist film 313 has beenprevented as described above, the resist pattern 313A can be accuratelyformed.

Next, as shown in FIG. 21(B), the p-type Al_(0.1)Ga_(0.9)N clad layer312 is etched by e.g. a reactive ion etching using a Cl gas plasma whileusing the resist pattern 313A as a mask, whereby a ridge structure 312 ais formed. As the p-type Al_(0.1)Ga_(0.9)N clad layer 312 isside-etched, the ridge structure 312 is trapezoidal in shape.

The resist pattern 313A is then removed by an organic solvent and thelike as shown in FIG. 21(C). Thereafter a p-electrode 314 with athickness of about 1 μm composed of a multiple-layer structure ofNi(nickel) and Au(gold) is formed on the ridge structure 312 a, as shownin FIG. 21(D). At the same time, an n-electrode 315 with a thickness ofabout 1 μm composed of a multiple-layer structure of Ti(titan) and Al isformed on the back surface of the GaN substrate 300. Since the backsurface of the GaN substrate 300 is made specular (see FIG. 18(E)), then-electrode 315 can be closely formed thereon without causing breaks orthe like. While not shown, the GaN substrate on which the nitridesemiconductor layer structures have been formed as shown in FIG. 21(D),i.e., a semiconductor wafer, is divided by cleaving, thereby completingthe nitride semiconductor laser device.

FIG. 22 shows a cross-section of the nitride semiconductor laser deviceproduced by the above-described method, i.e., the semiconductor deviceaccording to the seventh embodiment, along with the distribution of thelight-emitting region when the device is emitting light.

As the As impurity absorbs light with wavelengths of more or less 400nm, if the As impurity exists in the light-emitting region, the lightemitted by the nitride semiconductor laser device will be absorbed andthe luminous efficiency will drop. This problem, however, can beprevented in the seventh embodiment because, as shown in FIG. 22, thelight-emitting region 316 is not located in the light absorbing portions301 including the As impurity and therefore the emitted laser beam oflight is not absorbed.

Thus, in accordance with the seventh embodiment, since the lightabsorbing portions 301 are formed on the surface portion of the GaNsubstrate 300, the intensity of light incident on the substrate surfaceand then reflected by the substrate back surface can be reduced. Thismakes it possible to prevent the problem of exposing regions of theresist film other than the predetermined regions by the exposing beam oflight incident on the substrate surface and reflected by the substrateback surface in the photolithography step for the manufacture of thesemiconductor device using the GaN substrate 300. Accordingly, thepattern accuracy in the photolithography step can be improved andtherefore the manufacturing yield of the nitride semiconductor devicecan be improved.

In accordance with the seventh embodiment, since the light absorbingportions 301 are formed by the implantation of an impurity into the GaNlayer forming the GaN substrate 300, a drop in the crystallinity of thenitride semiconductor substrate having the light absorbing portions canbe prevented.

Further, in accordance with the seventh embodiment, the light absorbingportions 301 are formed by introducing the impurity only into theportions of the GaN substrate 300 below the predetermined exposureregion of the resist film. As a result, there is less of the impuritydiffused near the active layer above the GaN substrate 300 from the GaNsubstrate 300. This makes it possible to prevent e.g. an increase in theoperating current due to light absorption by the impurity, while thepattern accuracy in the photolithography step is improved and thereforeother nitride semiconductor layers other than the active layer can bepatterned at high yields.

In accordance with the seventh embodiment, since the back surface of theGaN substrate 300 is made specular, the manufacturing process of thesemiconductor device using the GaN substrate 300 can be simplified.

While in the seventh embodiment the light absorbing portions 301 wereprovided in the GaN substrate 300, a light scattering portion (see e.g.the first embodiment) or an anti-reflection film, i.e., a lighttransmitting portion (see e.g. the second embodiment), may be providedinstead.

While in the seventh embodiment the semiconductor device with a ridgestructure was produced using the GaN substrate 300, a semiconductordevice with a trench structure may be produced instead.

In the seventh embodiment, the type of the exposing beam of light usedin the photolithography step is not particularly limited. However, agreat improvement in the pattern accuracy over the prior art can beobtained by using light with certain wavelengths that can propagatethrough the GaN substrate 300 without being absorbed, such as g- ori-line, as the exposing beam of light.

While in the seventh embodiment, a positive-type resist film was use inthe photolithography step shown in FIGS. 20(C) and (D) and FIG. 21(A), anegative-type resist film may be used instead. In this case, the resistpattern is formed by using a photomask that covers regions other thanthe region (sandwiched by the light absorbing portions 301) where theridge structure is to be formed, and the resist film is developed so asto remove the portions of the resist film that were not exposed to theexposing beam of light.

Furthermore, while in the seventh embodiment GaN was used as thematerial for the nitride semiconductor substrate, this is only exemplaryand other group III nitride semiconductors such as GaN, InN, AlN ortheir mixed crystals may be used. In this case, the substrate mayinclude other materials as long as those group III nitridesemiconductors constitute a main component of the substrate.

1. A semiconductor device comprising; a semiconductor substratecomprising a group XII nitride as a main components, a n-type clad layerformed on said semiconductor substrate, wherein said semiconductorsubstrate has a light absorbing portion for absorbing a beam of lightincident on one planer; and a structure formed on said one plane of saidsemiconductor substrate by photolithography and etching of asemiconductor layer made of said group III nitride.
 2. The semiconductordevice according to claim 1, wherein said structure has a ridgestructure or a trench structure.
 3. The semiconductor device accordingto claim 1, wherein the beam of light is g-line or i-line of a mercurylamp.
 4. A semiconductor device comprising: a semiconductor substratecomprising a group III nitride as a main component, a n-type clad layerformed on said semiconductor substrate, wherein said n-type clad layerhas a light absorbing portion for absorbing a beam of light incident onone plane; and a structure formed on said one plane of saidsemiconductor substrate by photolithography and etching of asemiconductor layer made of said group III nitride.
 5. The semiconductordevice according to claim 4, wherein said structure has a ridgestructure or a trench structure.
 6. The semiconductor device accordingto claim 4, wherein the beam of light is g-line or i-line of a mercurylamp.